To sum them up:
Blue/Gary: Typical always-one-step-ahead-of-you rival. He's such a braggart even though he never beats you, even after becoming Champion, and later after becoming a Gym Leader.
Silver: Jerkass cold-hearted rival, the kind whose guts you just can't stand because he's almost your opposite.
Brendan/May: Rivals only in the sense of meeting and battling you repeatedly. Not so good since s/he stops fighting you after your fourth match (you have only six badges at this time). I also wish you could've named him/her.
Wally: Sickly green-haired child (asthma) who you help in catching his forst Pokémon. He's not much of a rival either since you only fight him twice, once with only his underleveled Ralts.
Pearl/Barry: The worst kind. He's like Blue in his bragging and that he's always ahead, but unlike Blue he's a reckless idiot with a wild and annoying personality. Worst part: it runs in the family (his dad's the same way).
I like the first two best.