I can't say that I've gone into any game with doubts. I'll give any game a fair shake, but I'll glady turn my back on it if it doesn't give a fun experience. That said, my favorite systems are:
D&D 3/3.5E: (No kidding!
) I left 2E when it ceased to give me what I wanted from a game, but when I read what all was being changed I had a good idea that I would probably like the new D&D. Sure enough I did.
D20 Modern/Future: I love this game. WotC, along with other companies like The Game Mechanics, Ronin Arts, Adamant, and LPJ Designs, did a great job of creating a good, modular ruleset that allows a degree of believability that still conveys a feeling of heroics that I look for when I play games. I'm not one for a lot of heavy reality in my games. If I want a common man to be able to do things that most cannot, D20 will not disappoint. Too many times, game systems try to simulate real life and bog down the play, forgetting the aspect of heroics that draw many (if not most) to RPG's in the first place. D20 Modern kept the heroics and gave us enough reality to to keep the feel in check. (IMO of course.
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M&M: I guess I had doubts on this one. Super heroes games are so hard to do right. Designers walk a fine line between creating a playable game that offers not only an easy to run system with balanced characters and one with enough powers that play as closely as we see in the comics. Green Ronin did an excellent job with this system. M&M is hands down my favorite system for running supers.
Warhammer Fantasy Roleplay v2: I only played the original a couple times and liked it alright. I don't remember alot about it, but other than a few clunky rules it seemed pretty solid. The new version took away a lot of the clunkiness and made as extremely playable (if not brutal at times) game. Very fun.
Call of Cthulhu D20: I never really cared for Chaosium's CoC system, but I like what was done with the D20 version. It's still scary, still deadly, but at least the PC's have a shot at survival now. (The old CoC campaign I played in was akin to a meat grinder. Not sure if this was due to how the rules were written or the GM, but I created a LOT of characters during that short-ran campaign.) This is the system I use for creepy, lethal campaigns even when I'm not really running CoC.
D20 Star Wars: I didn't much care for the WEG rules, and while I liked the D20 system I was skeptical that it could deliver on the SW feel. I had no doubts after the first session. The characters (all level 1) twarted an assassination attempt on the Supreme Chancillor (before Palpy took the role), had a daring chase via aircars through the skyways of Coruscant, and faced down a dark Jedi in the depths of the Undercity. Very cool. Very evocative. Great fun.
Savage Worlds: Picked this up on a whim, and I really like it. It's quick and easy to play and has plenty of character options. I don't like it as much as D20, but I would run an extended campaign with this system.
Deadlands: I love westerns. While Deadlands isn't really a true western ruleset (Sidewinder: Recoiled is what I use for true westerns), it's a great setting for creepy zombie-blasting, undead hunting in the wild west fun. Plus, I love the mechanics of playing cards in a western setting.
Hackmaster: I don't look at it as a serious attempt to make a "more logical" 3E than 3E is. I look at it as tongue-in-cheek AD&D. It's great fun in small bursts, and pretty funny to. Too bad the cost to DM's is so high with all the extended monster book series.
There are other systems that I've really liked over the years, but these are the ones that come to mind instantly.
Kane