Our community is so large and diverse that I believe one would have to reference both the flight sim and the genre of flight simulation before the addons would be meaningful.  Also, there are essential addons for certain genres and those which are just great to have.

P3DV4

Weather Engine

Certainly a weather engine is a first stop, and while there are some great freeware engines available (and they are very, very good) one really can't beat ActiveSky, at least not for present day.  This may well change in the near future though.

Aircraft

PMDG is always up there, but the Majestic Dash 8 Q400 Pro/Training Versions occupy the 1 or 2 slot. The other aircraft (again, 1 or 2 slot is the soon to be released Leonardo Maddog X (no release date at present, but work is going very fast on this incredible addon).  If you're a Boeing guy, then you're not going to beat PMDG for quality. The Ifly Boeings are a very good and less expensive alternative.  As a special note... the Shared Cockpit genre is going mainstream and I believe that we'll see a lot of titles becoming shared cockpit capable in 2018 and we'll get to the point where shared cockpit is factored into purchase decisions (please don't ask me beyond that as I can't provide more info for the time being).

'

If Airbus is your thing than for the moment you'll want the FS Labs A320, however for my money the Aerosoft Airbus A318, A319, A320, A321 is the way to go. I'm flying those aircraft in Beta right now and I'm loving them.  Can't avoid mentioning the A330, which will be hot on the tail of the baby buses.

Airport Addon Scenery

Anything produced by FS Dream Team, Fly Tampa and Flight Beam is a winner, though as things stand now I have to say that my favorite airports are Fly Tampa Amsterdam and FSDT Memphis.

Weather Textures

I placed this away from Weather Engine because when cost is a factor one can hold off on adding weather (cloud) textures or they can use one of the several high end freeware texture sets which exist in the community.  With that in mind, I'm personally a fan of EVENTEX but I have a sharp in on REX Sky Force which is rumored to be a real and true game changer.

Ground Based Services

Nothing touches GSX!

 

Best wishes!

 

Tough to narrow it to 5 choices. I'll try:

• FSUIPC
• AS16 + ASCA
• Orbx (Global and LC's and Vector is nice too)

only 2 left...

• PMDG aircraft (pick one, all good)
• FSL A320x

 

- Orbx Global

- Orbx LC / Regions

- Flight1 GTN 750/650

- RXP GNS2

- GA aircraft by a2a, Carenado, RealAir..

• ActiveSky for P3Dv4
• Chaseplane
• FSUIPC
• Orbx Global
• ENVTEX

(FSUIPC always) - ASP4 - My Traffic Professional - PMDG 777x - Navigraph -  VFX Central , my top five addons.Then follow the rest:  Super Traffic Board - PTA - EditVoicepack. I have but I do not install ORBX and ChasePlane ,because they give me problems..Later I may be able to install it back ..

ORBX Global + USA & Europe LC & FTX Airports

ASP4 + SC & Freeware Textures by Kiwi

Editvoicepack

Ezdok

Realair Lancair & Turbine Duke with GTN 750/650, Maj Q400, and XP Learjet 25

I spend more time tweaking these days

But...

Realair Legacy v2 (installer coming soon!)

Alabeo Ovation

A2A Comanche

RXP GTN's

Pilot Edge (amazing online ATC service)

 

FlyInside
FSUIPC
PilotEdge
Active Sky
FSEconomy

Missing are any planes or scenery as well as a few other addons I would want but I'd say those would be my top five that I use on almost every flight.

Active sky for P3d 

orbx global, orbx au, orbx au traffic

REX4 TD with Soft Clouds Enhanced

PTA

pmdg 737ngx and Fs2crew reborn

Navigraph 

ORBX (Global, LC, Vector, Regions, and a only a few airports... that's the story I tell my wife, and I'm stickin' to it!)

ASP4/ASCA

Flight1 GTN 750/650

PTA

GA aircraft... RealAir Legacy and Turbine Duke, A2A, Alabeo Mooney Ovation

- AS+ASCA

- UT Live

- Chaseplane

- PTA

- Majestic`s Dash8 Q400

 

In no particular order.

Active Sky

PTA

Chaseplane 

Orbx

PMDG

* FSUIPC

* ActiveSky for P3D4 + freeware sky texrure from Kiwi

* Majestic Dash 8 Q400

* Orbx Global +

* Ezdok2

For me currently...

- Orbx Regions & Airports

- REX4 TD + Soft Clouds

- OpusFSI weather engine

- PTA

- Milviz Beaver

Though I'm about to embark on the PMDG 737 and am eyeing ENVTEX but patiently waiting on REX's next product lineup.

Honourable mention to LittleNavMap.

ASP4 + ASCA + EnvTex 

PTA

Fsuipc

Fsps 3D Cockpit Sounds

OpusFSI ( buttkicker )

TSS aircraft sounds

RealTraffic + PSXseeconTraffic

Scenery : Orbx , photo real scenery , T2G , Dzrewicki Design , JustSim , FT , some AS