I just finished playing through Klonoa Heroes so I can list a couple annoying enemies from that game...
These three guys are pretty darn annoying with the blue one able to heal itself and other enemies nearby as it pleases. These guys are always placed in rooms with large amounts of enemies so they're triple the annoying unless you've got your special attack charged up before entering the area (or else the yellow one will begin draining your SP meter) or you're strong enough to take out many enemies with ease.
These guys are annoying unless you're strong and can simply kill them with two or less hits. The trick chests always make you afraid to even approach a treasure chest in case one of them is a trap.. then they chase you around and if they land a hit chances are it's critical damage and you have to break out the health items quickly. x_X The guys with the vacuums for mouths are annoying when you defeat enemies and said enemies drop lots of cash and EXP crystals.. vacuum nose wastes no time sucking it all up before you can grab it all. Kill the vacuum guys first!
Then.. there's the Sasami's. No matter what level I was at, no matter what equipment I had for my characters.. these guys could tap me on the side and cause me fatal damage easily. Those grins.. they know they're annoying. And they know I hate them...