Thanks! Most of those I'm pretty familiar with. Viewtiful Joe I'd like to check out as I think I've only played the demo. Billy Hatcher--same thing, I think, although I'm not sure if I even tried the demo of that one. I know it was on the same disc as Viewtiful Joe. I'd like to check it out, though.
If you want somewhat more obscure, but still good, recommendations I can provide those too. Be warned, these are not necessarily cheap.
Odama - Requires the Gamecube Microphone. One of Gamecube's more unique titles and probably the cheapest title on this list, it is part pinball game part real time strategy. This game is so bizarre it's definitely worth checking out for it's price. It's a bit lacking in content and is pretty damn difficult, but after my rage at the difficulty (and the fact it mocks you for losing) had subsided, I had a weird smile on my face.
Bomberman Generation - Overlooked sequel to Bomberman hero. Has classic Bomberman multiplayer packed in too. Exclusive to the 'cube.
Lost Kingdoms - EH. I'm not the biggest fan of this game, but it's definitely intriguing. Its a card based action RPG...That's the best way I can describe it. The story seems thrown together at last minute but the combat is interesting for sure. The first game in the 2 part Gamecube-only duology is cheap and also the only one I've played. They're apparantly short, so I'll get around to the more expensive (and supposedly better) 2nd game at some point. I only really recommend this as an oddity, though I know some people like it.
Baten Kaitos Origins - I feel like in the past year or so the Baten Kaitos Duology has got the renown it deserves because it's a Monolith Soft Game. I love the first game, but I acknowledge that it's overly cheesy dialogue and cliche plot aren't for everyone. Not to mention I know some people found the battle system somewhat stale halfway through. I'd still recommend it, but definitely only for certain people. Origins on the other hand is one of the great RPGs of the generation. The battle system is unique and sublime, the story is interesting and heartbreaking and the pre rendered backgrounds are still beautiful to look at to this day.
You CAN play this game before the 1st in the series because it's a prequel and if you have no interest in the other game, go ahead and do just that. I'd argue though, that the game is best played after the first one (you're not missing too much though) so if you have even a slight interest in the series, grab both and play them in that order.
Cubivore - Another unique cube title. Truly? Not worth the price it goes for, but it is a decent, if not repetitive, game. I could write more up about it if you're interested, but I guess I see it as more of a collection piece than something actually worth buying just to play it.
Ikaruga - Not really obscure but...If you only play one shoot 'em up in your entire life make sure it's this one. I'm not all that amazing at shmups, but this is a classic. In a weird way I think much of this game as I do F Zero GX. Yes they're completely different genres but they're both games that reward you heavily for high skill play and give the same type of satisfaction when you overcome a challenge. Not super cheap, but the best way to play a physical copy of this game in on the 'cube. XBLA has a digital version if you don't mind not having the physical disc.
Imports
There are a couple of imports worth mentioning. To play these, get homebrew on your Wii or a modded cube OR an action replay/freeloader disc for your cube.
Dream Mix TV World Fighters - An interesting Smash-clone. Unlike Playstation all stars which tried to be more complex than smash, this tries to be a whole lot more simple. Fewer moves and a pretty straight forward way to win, it's definitely pretty cool. Features some of the most awesome characters ever. Try pitting Optimus Prime against Solid Snake in any other game.
Hudson Selections - A collection of 4 polygonal remakes of Hudson games including Bonk and Star Soldier. If that's your thing, they're pretty cool.
Doshin the Giant - Giant simulator. You play the local friendly giant and help out the island locals...or destroy them, whatever floats your boat. It's pretty repetitive but I found it to be a bit of fun, especially when I had my girlfriend laughing at all the stupid things that happen in the game.
Speed Challenge: Jacque Villaneuve's Racing Vision - Keep in mind if you get this or the PAL version of Doshin, you're gonna need a TV that supports PAL resolution or someway to force them to a different resolution. This is a pretty interesting racing game. It controls kind of funny, but once you get used to it there's a lot of interesting quirks to this game.