Updated: June 1st, 2016
Hello Gents;
Below is a list of payware addons that are in my opinion a must have for FSX/P3D. I will only include payware products that are FPS friendly and work well on my system. Please feel free to agree/disagree, add to the list in the comments below. In addition, please keep and eye out for this tread, as more and more products will be added to this list. I hope that this list was somewhat helpful to you and made your decision on purchasing some items easier. Thank you.  

                   Aircraft *non GA:
Without a doubt PMDG. Probably one of the most expensive add-ons that are currently available. Before buying, PLEASE make sure you have selected the correct version of your simulator, as FSX/SE, P3D are two separate products and require individual purchases. 
PMDG 737 P3D* all versions  https://www.precisionmanuals.com/pages/product/P3D/ngx8900.html
PMDG 737 FSX/ FSX SE   https://www.precisionmanuals.com/pages/product/FSX/ngx8900.html
PMDG 777 P3D *all versions  https://www.precisionmanuals.com/pages/product/P3D/777LRF.html
PMDG 777 FSX/FSX SE   https://www.precisionmanuals.com/pages/product/FSX/777LRF.html

Majestic Q400. One of the best, if not the best add-on aircraft for FSX/SE/P3D *all versions. 
http://majesticsoftware.com/mjc8q400/

Aerosoft Airbus (318-319-320-321) 
Bundle: http://en.shop.aerosoft.com/eshop.php?action=article_detail&s_supplier_aid=13235&s_design=default&shopfilter_category=flight%20simulation&s_language=english
*Aerosoft Airbus is also available for sale in a packet of two only (i.e. 318-319 only, or 320-321)

If you are looking for something which is not so complex, you might want to consider the QualityWings 757. 
http://www.qualitywingssim.com/store.html

Aircraft GA: 
A2A Cherokee http://secure.simmarket.com/advanced_search_result.php?keywords=A2A+Cherokee
*Please note, same as PMDG, A2A has two different prices for P3D and FSX/FSX SE. 

RealAir Duke v2 P3D                  http://secure.simmarket.com/realair-turbine-duke-v2-p3dv2.phtml
RealAir Duke v2 FSX/FSX SE     http://secure.simmarket.com/realair-turbine-duke-v2-fsx.phtml
RealAir Lancair Legacy V2 P3D   http://secure.simmarket.com/realair-lancair-legacy-v2-p3d-v2-v3.phtml
RealAir Lancair Legacy V2 FSX/ FSX SE  http://secure.simmarket.com/realair-lancair-legacy-v2-fsx-fsxse.phtml

Scenery: 

The guys over at ORBX make absolutely great scenery, you cant go wrong when buying scenery from them. For me, the "must-have's" are the following: 
Orbx Global Base: http://www.flightsimstore.com/product_info.php?products_id=2542
Orbx Vector *although could be FPS heavy in some areas  http://www.flightsimstore.com/product_info.php?products_id=2712
Orbx OpenLC Europe: http://www.flightsimstore.com/product_info.php?products_id=2973
Orbx OpenLC USA/Canada: http://www.flightsimstore.com/product_info.php?products_id=3507
Orbx Trees *probably one of the best investment in FSX/P3D that actually increases performance over the default.  http://www.flightsimstore.com/product_info.php?products_id=3301
If you would like to learn more about Orbx and what are the main differences between OpenLC/Global/ Vectors please visit this link: https://fullterrain.com/

FlyTampa Airports FSX/P3D
FlyTampa Copenhagen*a must have http://secure.simmarket.com/flytampa-copenhagen-fsx-p3d.phtml
FlyTampa Dubai Rebooted *a must have http://secure.simmarket.com/flytampa-dubai-rebooted.phtml
FlyTampa Corfu http://secure.simmarket.com/flytampa-corfu-fsx-p3d.phtml
FlyTampa St Maarten Complete v1.8 http://secure.simmarket.com/flytampa-maarten-complete.phtml
FlyTampa Vienna http://secure.simmarket.com/flytampa-vienna-loww-v.2.phtml

Taxi2Gate Airports: 
Hong Kong International  http://secure.simmarket.com/taxi2gate-hong-kong-international-airport-vhhh-fsx-p3d.phtml 
Istanbul * a must have http://secure.simmarket.com/taxi2gate-istanbul-atatuerk-airport-ltba-fsx-p3d-p3dv2.phtml
Orlando International Airport http://secure.simmarket.com/taxi2gate-kmco-orlando-intl-fsx.phtml
Munich International Airport *a must have http://secure.simmarket.com/taxi2gate-munich-airport-eddm-fsx-p3d.phtml
Seattle-Tacoma International Airport http://secure.simmarket.com/taxi2gate-ksea-seattle-tacoma-international-airport-fsx.phtml

UK2000 Airports FSX/ FSX SE/ P3D
Belfast Xtreme http://secure.simmarket.com/uk2000-scenery-belfast-xtreme.phtml
Edinburgh Xtreme V3 http://secure.simmarket.com/uk2000-scenery-edinburgh-xtreme-v3.phtml
Gatwick Xtreme http://secure.simmarket.com/uk2000-scenery-gatwick-xtreme-v3.phtml
Glasgow Xtreme V3 http://secure.simmarket.com/uk2000-scenery-glasgow-xtreme-v3.phtml
Liverpool Xtreme: http://secure.simmarket.com/uk2000-scenery-liverpool-xtreme.phtml
Manchester Xtreme http://secure.simmarket.com/uk2000-scenery-manchester-xtreme-egcc.phtml
Newcastle Xtreme: http://secure.simmarket.com/uk2000-scenery-newcastle-xtreme-egnt.phtml
VFR Airfields Volume 1 V2 *great value for money http://secure.simmarket.com/uk2000-vfr-airfields-volume-1-version-2.phtml
VFR Airfields Volume 2 v2 *great value for money http://secure.simmarket.com/uk2000-vfr-airfields-volume-2-v2-fsx-p3d.phtml

Aerosoft Airports FSX/P3D:
Ibiza X Evolution http://secure.simmarket.com/aerosoft-ibiza-x-evolution-fsx-p3d-(download).phtml
Mega Airport Lisbon v2.0 http://secure.simmarket.com/aerosoft-mega-airport-lisbon-v2.0-(download)-fsx-p3d-p3dv2.phtml
Mega Airport Prague http://secure.simmarket.com/aerosoft-mega-airport-prague-fsx-p3d.phtml
Mega Airport Frankfurt V2 http://secure.simmarket.com/aerosoft-mega-airport-frankfurt-v2-fsx-p3d.phtml
Mega Airport Rome http://secure.simmarket.com/aerosoft-mega-airport-rome-(download)-fsx-p3d.phtml

DrZewiecki Design Airports: FSX/FSX SE/P3D
Krakow: http://secure.simmarket.com/drzewiecki-design-krakow-balice-epkk-2012-fsx.phtml
Warsaw: http://secure.simmarket.com/drzewiecki-design-epwa-warsaw-chopin-airport-x-fsx-p3d.phtml
Moscow: http://secure.simmarket.com/drzewiecki-design-uuee-moscow-sheremetyevo-x.phtml

Tools/Utilities 
FSUIPC http://www.schiratti.com/dowson.html  ****Absolutely a must have (paywear version) 
Ezdok: http://www.flight1.com/products.asp?product=ezdockcam *a must have
Rex Soft Clouds http://secure.simmarket.com/rex-soft-clouds_fsx-p3d.phtml
Rex 4 Texture Direct: http://secure.simmarket.com/rex-rex4-texture-direct-hd.phtml
****For more information about REX and what it does, please visit the following site: http://www.rexsimulations.com/ ******************************
Active Sky Next *a must have  http://secure.simmarket.com/hifi-tech-active-sky-next.phtml
Aerosoft Professional Flight Planner X  http://secure.simmarket.com/aerosoft-professional-flight-planner-x-(download).phtml   *a must have
FS real Time http://www.3dsoftworks.net/products/fsrealtime/
Navigraph (charts, AIRAC) https://www.navigraph.com/