Super Mario Bros. - Much better. With my favorite game this generation being Super Mario Galaxy and with the 2D greatness of New Super Mario Bros. (both Wii and DS)... it's a great time to be a Mario fan.
Sonic the Hedgehog - Slightly worse. It seems that Sonic has started to rely on way too many gimmicks this generation, but at least Shadow has taken a back seat. The Rush series (DS) is great though, but even that can't keep up with the wonderful Advance series.
Smash Bros. - Better. First of all, Sonic's in the game for Pete's sake. Not to mention Toon Link... great times!
The Legend of Zelda - Slightly better. It's tough to keep up with the stellar Wind Waker, but Twilight Princess is another fantastic entry in the series. Also, the DS games (Hourglass and Tracks) might not be my pinnacle Zelda games, but they're the first portable Zelda games that I've started and finished. I'm a better gamer for it.
Castlevania - Better. Dawn of Sorrow was a wonderful way to start out the generation, and even though Portrait of Ruin couldn't keep up the pace of previous titles, Order of Ecclesia proved to be a truly wonderful game. I'd say that these three DS games slighty edge out the three GBA games.