1) Bandits. Very versatile, with only a couple of annoying abilities (Divine Shield). Can be used for many triggered events (transmissions, ambushes) to make enemy encounters feel more alive.
2) Gnolls. Not as versatile as bandits but are satisfying to kill, can be used on many maps and don't have any annoying abilities (like Magic Immunity, Slow, Flying, Possession).
3) Murlocs I guess
I hate trolls. Every troll feels the same unless it's a Priest.
Golems are OK, but the larger ones are annoying when you can't use spells on anything else.
Infernals for me. The first time I saw it I'm immediately have "oh sh*t" sense in my mind. They are freakin strong, especially with that Chaos damage type. Dealing with them is always a pain in the campaign.
Maybe if Land Frost Wyrm as a ground unit creep existed -- like in those old pictures of how it was originally going to be on Warcraft III -- then maybe this would be the coolest monster. "Cool" as in cold magic and also "cool" as in most awesome, don't you think?
In general just being dragonlike makes something cool. I wonder if you gave Thunder Lizard wings and made it fly like a dragon if it might become more cool?? Or is that even possible?