this is a hard one, but i think i have to go with Tecmo Super Bowl on my old NES. man i played that game a lot and i still do play it. as far as fun factor, it's the best football game ever made.

OMG what a question... ummmmm... urrrrrrr...

of my youth; super mario kart
of my pc beginnings; quake
currently; too hard to say, most games are of such a quality nowadays that i rarely spend any length of time playing any one game, right now this second i have 'star trek - bridge commander' on my quick launch bar.

for sheer re-play-ability, addictiveness, and lost hours, not to mention one proper girlfriend telling me i care more about my make believe football players than her and me replying your right now f**k off and don't slam the door on the way out <<< true story , the all time classic must be CHAMPIONSHIP MANAGER, now known as FOOTBALL MANAGER.

GT Legends actually GTL-GTR2 as it adds more options, then GTL.

It just has more cars that you can put your brush on, that's all.  

 Actually, you're close. Evo's GTL version is a bit of a pain to mod cars for. Rims & such. And both conversions still lack sunalp2's LHD conversions...

 I do like Evo but prefer GTL & GTR2 for variety and Evo for ffb feel.. Hardly ever touch rfactor. haven't in about 4 months?

Ultima Online (Pre tram)
Star Wars Galaxies (pre sony buyout)
EvE Online

all of these games had/have awesome PvP (i do not like AI ... AI can always be beat one way or another a human LEARNS and adapts)

All the simulations rFactor .. Il-2 (just got but boy am i having a blast doing a campaign right now) Race, GTR etc etc are awesome fun when racing and such online

Xbox Forza Series is just amazing ... if they could come out with a Forza for the PC (ranks customizations etc) i would never ever leave my house and prolly die in 2 months from lack of food and a life haha

FPS ... Call of Duty series for sure wins with Arma being a close second for realism

RTS ... Company of Heroes is good Starcraft is good ... Men of war is bad ass

Sports: Madden was good now its just meh ... NHL (ea sports) is a ton of fun online now with the be a pro mode ... my friends and I each play our normal positions and make our pro as close to possible as real life (me im fast with some nice moves (dangler) and a slow but accurate shot) it is intense online with 12 people playing

that is what i can think of now ...

However both games went south and went "carebear" 

i think the oldest ones i go back to over and over again are . green beret (russian attack), chop lifter . after burner , out run .just love those classics . more upto date my favourate is prob graw and graw 2 on xb360 and on pc im still getting to grips with il-2 but that will be in there  and farcry no 1.