ff: crystal chronicles : haven't played it either, but majority of people say that you are missing out if you dont play it multiplayer, and most people dont have the multiple gbas and cables to play multi.  also, from what i have read, it is more of an "action rpg" rather than the traditional ff rpg series.

battalion wars: personally i thought it was a very fun and refreshing game to play.  i have never played the advance wars gameboy series, so wasn't really dissapointed, and with it only being less then 10 bucks new at gamestop, you dont have to dish out too much for an entertaining game.  however, i must admit the controls do require a bit of getting used to, and there is a lack of immediate replay value.

lotr: third age: i think this game is pretty fun and entertaining.  it plays like a very traditional turn based rpg, with skill trees and such.  i am not a real big rpg gamer, so i dont really know how it stacks up to the other rpgs, but i liked it.  also some pretty good graphics.  if you only have a gc, then i recommend it, as it is one of the better gc rpgs.  however, the story is a bit cheesy, hehe. 

pikmin 2 is coop, pop:ttt isn't

if you like action rpgs or are into comic books, xmen legends (1&2) are both fun (w/ up to 4 player coop).

straight up action, lotr: return of the king is also a good choice

fps, timesplitters 2, timesplitters future perfect are also great (reminiscent of goldeneye and quirky)