My idea of what "a system" means might well be more broad than other people's...
If you wonder, it's based on the degree to which I'm prone to mix and match them while running them.

1. Runequest/Mythras, Pendragon and all the d100 systems like Maelstrom, Haunts and Horrors and Unknown Armies (and possibly the upcoming RuneQuest, but we'll see about that).
2. BoL, Traveller, Fabled Lands and the Chainmail-inspired OSR games (Spellcraft and Swordplay, Crimson Blades, Dungeon Questing, Five Ancient Kingdoms, Tales and Legends, possibly others that I'm missing), along with Fantasy Age.
3. Pocket Universe games and games based on the same engine (Bethorm very much included) have taken the place of GURPS for me.
4. Spellbound Kingdoms really is its own system, but it's fun and makes sure that even the included narrative elements make IC sense.
5. The setting/genre-based OSR, like Tunnels and Trolls, Earthdawn and the OSR of the 3rd wave, like Crypts and Things, DCC, The Nightmares Underneath, Low Fantasy, Scarlet Heroes, ACKS (the last one is between the 2nd and the 3rd wave, but whatever).

Yeah, it's a surprisingly lengthy list, I know;)!

Quote from: Black Vulmea;756743In no particular order.

• Original, 'classic' Traveller
  
• 2e box set Boot Hill
  
• Top Secret
  
• Flashing Blades
  
• Metamorphosis Alpha

Honorable mention: Marvel Super Heroes, d20 Modern, Mutants and Masterminds

Aaaaaaand two-and-a-half years later? No change.

Quote from: Simlasa;756702No particular order after CoC:
Call of Cthulhu (and various other iterations of BRP)
WFRP
Classic Traveller
GURPS
Gamma World (original)

Nowadays DCC would edge out GURPS for me, and Magic World is my primary source of BRP goodness over CoC.

Quote from: jeff37923;756707Traveller (Classic/Mongoose/4th/5th)
2300AD (both)
WEG d6 Star Wars (all)
Mekton (Zeta/II)
Basic D&D/Labyrinth Lord

2300AD (both) has dropped to last place, but everything else is still the same 2.5 years later.

1- Dark Heresy
2- Deathwatch
3- D20 Modern
4- Pathfinder
5- D&D 5e

Yeah, I know.  I'm not an OSR guy.  I might get DCC soon, though.

Lamentations of the Flame Princes (B/X D&D)
Classic Traveller
King Arthur Pendragon
Sorcerer and Sorcerer & Sword
Hero Wars in Glorantha

There's a spread of distinctly differently kinds of gaming across the list. I like each one for what it offers that the others can't.

Damn, this is hard. I own and enjoy too much shiznack.

1st - Whatever I am writing at the moment (whether house ruled versions of Game X or my own stuff)
2nd - My version of OD&D (very much akin to S&W: WB)
3rd - Classic Traveller (including my 40k crossovers and houseruled whatevers)
4th - Gamma World (either classic or the 4e boardgame version, both heavily houseruled)
5th - Call of Cthulhu (getting juiced these days by both Silent Legions and Amazing Adventures)

Of course, I gotta caveat that I'd toss aside my list if a GM I trust is running whatever else. If I know somebody brings the kickass to the table, I will play pretty much anything outside my pet peeves (I can't do 3e anymore and I don't enjoy narrative RPGs) and I may even set aside those if I know the GM well enough.

1. Fighting Fantasy Books (I started with these)
2. Shadowrun (love the universe)
3. Warhammer 40K (love the universe)
4. D&D (all versions), honourable mentions: 13th Age, DCC
5. Dragon Warriors (simple and awesome)
6. sorry! Low Fantasy Gaming RPG

Quote from: kobayashi;760120The 5 games that gave me my best campaigns so far :

Warhammer 1e
Legend of the five rings 1e
Savage Worlds
The Laundry
Waste World

So two years later...

Waste World is still here because it rocks. Stills feels like the best unofficial 2000AD comics rpg.
Chroniques Oubliées: a french OGL fantasy game that is a perfect fit for me and my players. Light but with enough toys for the players to play with. A bit like Radiance.
The Laundry is still here as well, a very refreshing take on the Mythos, mixing horror and humor perfectly.
D20/OGL : I know it's a bit vast but from Star Wars to Call of Cthulhu, Mutants & Masterminds, Grim Tales, Fantasy Craft, Microlite games... Not any game that I would play as is, but there's a lot to plunder for my own creations.
Lamentations of the Flame Princess : kinda replaced Warhammer 1e as my go to gritty fantasy game.


Why ?
Warhammer 1e : own almost all the books, but I wouldn't play the game with its original rules now.
Savage Worlds : shaken status, wounds, bennies, railroady campaigns... I had a ton of fun with it but it was time to let go.
Legend of the Five Rings 1e : no game group interested in it.

In no particular order,
Basic Fantasy RPG
RIFTS
Stars Without Number
Exalted 1e
B/X D&D

Honorable mention: Mutant Chronicles

Blades in the Dark
Star Wars (FFG)
Cypher System
The Sprawl
Fading Suns

Quote from: CRKrueger;837826In no particular order...

Runequest (6th, MRQII)
Shadowrun (2nd, 3rd)
D&D (1st Ed, Basic, 5th Ed, D20 Conan)
Rolemaster (MERP, RM2, RMSS)
WFRP1
Rifts

Those are the systems I've played the most and got the most enjoyment out of (as I currently remember it ).

Honorable Mentions: Traveller, FASA Trek, Earthdawn, OpenQuest/Renaissance, DCC, ACKS, AS&SH, Dragon Age, 2nd Ed, Gamma World, Top Secret, Aces & Eights, Mythus, oWoD, Hackmaster 5th, Cyberpunk 2020, Star Frontiers.

Yep pretty much the same.  You twist my arm to get to 5 and don't allow ties...
Mythras
Shadowrun
AD&D
Rolemaster
WFRP1

Pathfinder/D&D 3.5
Fate
Gumshoe/Nights Black Agents
Traveller (Mongoose and Mega being my favorite Varieties)
Fantasy Craft

1. D&D (Original D&D or 1e AD&D)
2. BRP/Call of Cthulhu
3. BRP/Runequest or Stormbringer
4. Rolemaster (2)
5. Behind Enemy Lines (Fasa)

The ones that gave me more hours of fun, in no order of preference:

Vampire the Masquerade
Shadowrun 2e/3e
Apocalypse World

The ones I appreciate from reading (or playing a little) and would like to play much more:

Runequest 2e/6e
Pendragon
Unknown Armies
Sagas of the Icelanders
Shinobigami