For various in depth takes on this, see The 12 RPGs Every Gamer Should Play Before They Die (Gnome Stew), Ten Games You Have To Play Before You Die (Geek Related, me!)

I remember going through the process of breaking out of the D&D Ghetto (tm) into other games, and certain games did a lot to expand my understanding of what you can do with the field. Here I'll give shout outs to the big three.

• Call of Cthulhu redefined heroism and stressed smarts over violence and has a downward spiral instead of a level climb.
• Amber, way before any of the recent crop of indie games, demonstrated that you didn't need much in the way of rules and could have a game that was almost completely social.
• Feng Shui, showing you can break out of the "level appropriate challenge" grind and fetch quests into being a real movie style badass, and do genre emulation.

For various in depth takes on this, see The 12 RPGs Every Gamer Should Play Before They Die (Gnome Stew), Ten Games You Have To Play Before You Die (Geek Related, me!)

I remember going through the process of breaking out of the D&D Ghetto (tm) into other games, and certain games did a lot to expand my understanding of what you can do with the field. Here I'll give shout outs to the big three.

• Call of Cthulhu redefined heroism and stressed smarts over violence and has a downward spiral instead of a level climb.
• Amber Diceless Roleplaying Game, way before any of the recent crop of indie games, demonstrated that you didn't need much in the way of rules and could have a game that was almost completely social.
• Feng Shui, by Robin Laws, which shows you can break out of the "level appropriate challenge" grind and fetch quests into being a real movie style badass (freeing you up from having to worry about XP and loot and leveling and such), and do strong genre emulation in the bargain.

You can play a bunch more traditional games, but you really only get a little bit of insight from them. I'd try games that teach you how to do something different. IMO, Earthdawn teaches you how to be D&D with the serial number filed off. It's a good game but not different enough that you're going to gain a lot in terms of overall techniques from learning it - it's like learning C, C++, and Pascal instead of learning C, Lisp, and Erlang. GUMSHOE, on the other hand, teaches you an entire different approach to investigation games. There's a lot of good games, but if you want to experience the breadth of the hobby, you need to be picking the perimeter points.

As I've thought about it more, I'd definitely play a weirdo indie game - Fiasco or Blowback or whatnot, or even one of the more "super trippy even the GM isn't sure what's going on" indie games like Lacuna Part 1 (I see it's being re-released, it has a listing in GTM last month). If you have the stomach for more serious/adult, I might do Unknown Armies even instead of Call of Cthulhu.

People like Savage Worlds but again I don't know it will teach you anything (especially since you've played earlier less rules-bound D&D's like 2e). FATE - it's the new hotness, and it has some new-ish stuff, but there's other equivalent choices for "kinda trad but with narrative flair" like Cortex in Smallville or Unisystem in Buffy The Vampire Slayer.