On bosses in general, Pocket Chomps, hands down. Those are pretty fantastic.
I also use Bro/Ice Flowers and Shells a lot (on both bosses and regular enemies), but Shells only work on ground enemies. Mix Flowers are awesome, but I use those more in battles with multiple enemies/things to attack. Plus they're expensive or harder to come by. I use Smash Eggs every once in a while (took me a while to get the hang of them), but pretty much never use the rest of the items.
Typically my tactic for bosses is to use Bro/Ice Flowers to lower the enemy's stats, then use Pocket Chomps. <3