Microsoft Flight Simulator 2020 has turned into a beast with amazing addons that make this simulation extraordinarily realistic and fill the world with immersion. I would like to compile my personal favorites of mandatory addons that make airliner pilots cry of joy.

If you are into airliners, I highly recommend the following software:

• Fenix Airbus A320-200 (the most realistic study-level airliner on MSFS so far, it's an A320ceo with a never before seen simulation depth. A lot of love has been put into graphics and sound too. Examples: You will even see brake fans rotating. The GPU cart gets louder when more flight systems are turned on. This addon sets new standards.)
• flybywire A32NX 🆓 (the free open-source alternative to Fenix (actually it was first), an A320neo which is also study-level, and getting closer and closer in features. Everything flybywire does is highly sophisticated and looks awesome. What a professional team. This is the best open-source aircraft.)
• PMDG Boeing 737-700 or 737-800 (the -700 was released first so everyone bought it and there are double as many liveries available for it, but that may change because the -800 was the by far most popular 737 in real life. I like that the -700 has extended range and all newer systems of the -800 and -900 can be optionally plugged in. There's also a 737-600. PMDG has always been delivering top quality, but the 737 has seen quite a few bugs lately.)
• Inibuilds Airbus A310 🆓 (Inibuilds A310 is included for free in the 40th Year Anniversary update of Microsoft Flight Simulator. Simulating the A310 was a good choice, it has an outstanding cockpit and when it was released in 1983 it was years ahead. Simbrief integrated, almost study-level, huge potential. This is the one plane that justifies the price for Microsoft Flight Simulator Premium Deluxe Edition. I love it.)
• Fly the Maddog X MD-82 (the McDonnell Douglas MD-82 looks beautiful, it's sophisticated, it has incredible physics and it is complex. There is also a 16k cockpit texture overhaul which I recommend. This is such a detailled beauty and I love flying it.)
• JustFlight 146 Professional (The BAe 146 is a regional quad engine airliner from the 80s with short-field performance and quiet operation. This commercial addon has undergone a lot of research, it is full of details, there are so many hidden gems to be found in the cockpit. The cockpit is so well designed in real life, it is a blast flying it. Simbrief integrated.)
• Aerosoft CRJ Bundle (includes the Bombardier CRJ 550/700/900/1000 regional airliners. A lot of work has been put into this addon and I find it very enjoyable. I fly the CRJ 550 because it doesn't look like a long tube. Simbrief integration is also included. This was the first high level aircraft in FS2020, and it is still among the top.)
• Aerosoft Twin Otter (the twin-turboprop powered STOL aircraft “de Havilland Canada DHC-6” is available in 13 models. The DHC-6-100, -200, -300 can be flown as passenger, cargo and skydiver variants with wheels, tundra wheels, floats, skiers and amphibian. That makes this plane an airliner that can carry up to 20 passengers wherever you want. And it is an amazing product with smooth physics, everything just works. You will adore this plane if you add the DHC-6 Soundpack by FTSim+.)
• Working Title CJ4 🆓 (the Working Title CJ4 is an improvement mod for the default Citation CJ4 that comes with the Microsoft Flight Simulator. That includes a Rockwell Collins Pro Line 21 FMS represented at over 75% accuracy and major functional and visual enhancements to the PFD/MFD. Simbrief and Navigraph integrated. This is an outstanding open source mod, making it one of the best planes.)
• PMDG DC-6 (the Douglas DC-6 is a piston-powered retro airliner. If you want to fly this incredible beast you will also have to study navigation, it's an outstanding classic airplane that requires a lot of learning.)
• simbrief.com 🆓 (opened in your web browser, an industry standard pre-flight application for generating flightplans and sending them to FMS/MCDUs of your aircraft. You will need Simbrief Downloader 🆓 for PMDG, Aerosoft CRJ and Maddog MD-82 aircraft. The Fenix A320, flybywire A32NX and Salty 747-8i can pull directly.)
• Navigraph Navdata Center & Simlink (up-to-date navigation data for your simulator and addon aircrafts plus outstanding Boeing Jeppesen charts for flight planning and live navigation, requires a monthly Navigraph subscription, but it's really that good. There is also a Navigraph In-game panel, but I like the web version at charts.navigraph.com better if you have two screens. That website allows to easily create detailled flightplans. The flybywire A32NX has Navigraph integrated in its tablet.)
• Little Navmap 🆓 (it's a free flight planner, navigation tool, moving map, airport search and airport information system with Simbrief integration. It's a massive tool for creating or fine-tuning flightplans, even better than Navigraph itself. To get updated Navigraph airac cycles into Little Navmap you will need yet another Navigraph tool: FMS Downloader)
• Aerosoft Simple Traffic (highly optimized real liveries for all commercial AI aircraft in online mode, finally you see Hawaiian Airlines at Honolulu etc. In offline mode, airplanes are injected from included flight plans. It works transparently, it auto-updates, I love how it's fire and forget for a small price tag. Does not integrate with virtual ATC.)
• FSLTL 🆓 (as a free alternative for Aerosoft Simple Traffic, this one is manually updated via the FlyByWire Installer. The injector has to be manually started. AI traffic has no nametags (unlike Aerosoft Simple Traffic). It can pull directly from FlightRadar24, it has more than 1100 liveries already included, it works with Vatsim for model matching. Exclusive feature: Detects if FSHud is running and if so, let's FSHud inject the traffic and control it via ATC.)
• FSHud (Enhanced air traffic management that brings more convincing ATC functionality and accurate traffic separation. Finally you can load a simbrief flightplan into ATC, even in-flight. Supports custom approaches and let's you change runways and parking spot. Integrates with FSLTL.)
• FSDreamTeam GSX Pro (Realistic ground services for all airplanes, you will even see passengers boarding your airliner. This product is constantly updated, an unbelievable amount of work has been put into this. I don't understand so many people ranting about it, because I believe this addon is absolutely mandatory.)
• SLC (Self Loading Cargo, it is still the best passenger/crew simulation by far)
• MSFS Door Tool 🆓 (you will need it for SLC to be compatible with FS2020: For boarding/unboarding you will make SLC detect a door has been opened/closed. If you don't fly the Fenix, Toolbar Pushback also does this job. Find this addon below.)
• FSUIPC7 🆓 (you will need it for SLC to communicate with the sim. Others mostly use it for connecting hardware gauges and controllers in their home cockpits.)
• Simrate Bandit 🆓 (when speeding up time in FS2020, there is no other way to know what current simrate you are at, wtf Asobo!)
• Toolbar Pushback 🆓 (only if you haven't bought the Fenix A320, it's known to cause severe issues with that aircraft. If you bought GSX Pro you won't really need it anyway)
• vPilot 🆓 (you will need this client if you play on VATSIM (and Altitude 🆓 if you play on IVAO) to interact with human ATC controllers and follow real life aviation procedures. I don't like the stress of trying to understand people who chew on their microphones while mumbling incomprehensible English as fast as they can. Since I don't get paid 8k a month by an airline company I mostly skip this torture. Real pilots think that not being on IVAO/VATSIM is arcade gaming. I can't answer to you which one of the two networks is better, VATSIM has a few more online pilots.)

Here is more highly detailled stuff that needs to be listed yet isn't mandatory:

• FlightShare (Great live mapping. It's small and fast. Plus it's the swiss knife for manipulating your flight in-game, it keeps you from waiting or restarting e.g. by allowing to teleport yourself to another continent. It also allows easy live flight planning of the autopilot on default aircraft. If you have a lazy sightseeing day or need to go somewhere really quick you'll love this tool. Just don't try to autoland with the autopilot, it's not accurate enough. Evaluation version requires payment after a few months.)
• Headwind Airbus A330-900neo 🆓 (If you would like to fly a larger Airbus, then this is currently the best option for MSFS. The beautiful Headwind Airbus A330-900neo comes integrated with a modified flybywire A320NX cockpit and thereby doesn't mess up your existing flybywire A32NX installation such as the LatinVFR Airbus A321neo currently does with the FBW Compatibility Mod.)
• Salty Simulations 747-800i 🆓 (It's an improved Asobo Boeing 747-8 until PMDG finally releases their Queen of the Skies. The 747 is my only love, hurry up! The CDU is neat and it has Simbrief integration.)
• Heavy Division B78XH 🆓 (this is a slightly improved version of the Asobo Boeing 787-10 Heavy which is free for Premium Deluxe customers of Microsoft Flight Simulator. This is the only next generation Boeing airliner available and while it's a limited default aircraft, this beauty is my favorite Asobo stock airliner. Simbrief integration is not finished yet, but flying a manually entered flight plan works fairly well. Causes some issues with other addons so I keep it uninstalled until I want to fly it.)
• FS2Crew (if you are a real pilot, this will change single pilot gameplay to multi-crew flying for the most popular studylevel airliner addons. You basically talk to your virtual co-pilot over the microphone, who will assist with all procedures.)
• Skyvector.com 🆓 (open in a browser, a free website for displaying and planning flight routes. FlightAware uses it to display routes too. If you don't have Navigraph and don't want any software installed then this is a helpful site for route planning. I use it mostly to get super quick info about airports and their runways.)

🆓 = This software is free of charge or there's a free version available with sufficient functionality.