I would say that the single green shell is a useful item seeing as you can shoot it backwards. The three green shells are awful though because you can't shoot them backwards, they don't home (sp?), and you end up just shooting them to get rid of them and they bounce off the walls and hit you. Pain in the arse, that's what they are.
Single green shell - not bad because you can use them backwards.
Three green shells - not bad as protection but aren't great for shooting - theres a good chance you'll end up hitting yourself.
Single red shell - very useful if you're near the back of the group - they go after the enemies, whereas using a red shell nearer the front can end up going into walls and the like.
Theree red shells - same as above, only 3 is better than 1!
Blue Shell - great fun - especially if you don't like the guy who's winning!
Single banana - can be great if positioned just before the jump on Wario Stadium, just before the last power-boost before the jump on Royal Raceway (although they should be avoidable there), just before the power boost on DK Raceway, on the ramp leading to the shortcut on Koopa Troopa Beach, or just on the turning before the power boosts on Royal Raceway - enemies can slip and end up behind the ramp.
Five bananas - Pretty much the same as above, but 5 are much harder to avoid when placed in the above positions.
Single mushroom - My least favourite - not much good unless you've just been hit by a shell, fallen off a bridge etc.
Three mushrooms - Good for cutting across grass.
Unlimited mushroom - Great for cutting across grass.
Star - candidate for the best one - destrtuctive when coming up against enemies and great to avoid being hit by anything. Great way to catch up.
Lightning - another candidate for best one - brilliant when used as your enemies approach the jump on Wario Stadium.
Ghost - great if your enemies have lightning or stars.
Block (upside down question mark) - brilliant when placed right amongst real power ups, especially on Toad's Turnpike.
Best: Star. This is the best one to get. To bad I rarely got it.The lightning was second best.These help the most.The red shells came in handy to.The green ones sucked though! They always trailed back and I got hit with my own shells.Banana's were useless and didn't help you at all.The mushrooms that helped with speed helped when I was losing bad.