Red: Missingno. No, seriously. Red holds yet another themed challenge of mine, in which I focus my training mainly on the training of Missingno. It's mainly for research purposes, namely the following:
1. To see how long it takes before my game becomes corrupted.
2. To see how well a Missingno does in battle.
3. To find out just what the Bird type is weak/resistant to.
So far, it's going great. My Missingno is at level 32 (mainly because I'm in the middle of the game, just before I hit Erika), and I've had no problems. On the other hand, it seems that Missingno also inherited Bulbasaur's stats and type quirks, which leads me to believe that all Gameshark did was give the Bulbasaur I chose at the beginning of the game a Missingno mask. >_>
Anyway...
Silver: None. I was going to start another themed team there, but it just never really happened.
Crystal: An Alakazam, if I recall correctly. That baby helped me through the E4 I don't know how many times. I think I left it in the 70's region (as in, it's level 70+), but I haven't touched my copy of Crystal in a long while.