I loved MP#3's Item system, especially the fact that you could hold 3 items in MP3 versus 1 in 2. Reverse Shroom and Boo Repellant Rule all. The Rare Items are cool too, especially the Koopa Card.
Can't say much about 2's system, since most of the items that appear here are in MP3 as well - the sole exception being the Bowser Bomb. That's a very WTF item, as in WHY would ANYONE WANT this? Funny when someone gets one, though.

I prefer Orbs the most though, especially the ones you use on the spaces themselves. They give a sense of ownership and the ill effects don't affect the user, as opposed to the Capsules, which don't discriminate who lands on or moves after them (which sucks if it's one where you lose coins - and you placed it on the board, and placing Mushrooms and Wigglers on the board is just... WEIRD).
Mini-Mega is interesting, and the Zap-sticker is the grandfather of the orb/capsule system. Beyond those, there's nothing much to write home about the items in MP4.