The 3d (ie non-GBA) Custom Robo games are the best robot-fighting games I have ever played. They're good games thanks to the great combat system -- as usual with Nintendo, the games are great because of their great gameplay, certainly not for the stories (though that twist near the end of the GC game is interesting). The interaction between the three different weapon types is really key, it's a really great design. And of the Custom Robo games, the GC game is definitely the best one because it's the only one with 3 or 4 player battles, and they're awesome! It's really unfortunate that Nintendo let such a great series die, it should come back.
It takes a while to get going, but has one of the best, craziest, most fun and very pixar-like stories ever to be in a game.
I think of the game as Animal Crossing meets Toy Story meets Wall-E.
The concept of a small robot in a big world is a good one. From the Micro Machines racing game series to Toy Commander on Dreamcast, it's a concept that has been done very well before... but Chibi-Robo isn't one of those games. It's far too boring to be compared to greats like those.
As an aside, Animal Crossing, there's a series I have never played. It looks like The Sims but worse and even more limited, and The Sims is so boringly uninteresting looking that I['ve never played one of those games either; I love SimCity (particularly 2000), but when they changed from a focus on building things to a focus on managing people they lost me.
I do like Toy Story and Wall-E (Wall-E is amazing, and easily Pixar's best movie), but Wall-E didn't have to go plug in every thirty seconds, and the movie wasn't all about him picking up garbage until you get bored and turn it off.
What's everyone's favorite third-party game? Mine is a tie between Twin Snakes (don't judge me) and Soul Calibur II.
That's easy, Skies of Arcadia Legends! That game is absolutely amazing, and is my favorite JRPG ever. Incredible classic.
#2 would be XGRA: Extreme-G Racing Association, because I really love that game. Rogue Squadron II: Rogue Leader would be in third (technically it's not published by Nintendo, even if Factor 5 and Nintendo were close...).