Glow Squid.. I was mad when they got in because I wanted Moobloom.. But they look so pretty!

Bats, with their cute little squeaks

Tameable: Cat (I'm going for the Complete Cat-alogue advancement.)
Neutral: Enderman (I've always found them cool and mysterious. I never go to the End because it feels wrong to kill them and invade their home)
Hostile: Ghast has usurped Creeper for the time being thanks to the upcoming content drop
Boss: Ender-Dragon (can't go wrong with the OG)

i have this name for a reason

Wither ig

creepah

cat i love cats

Good question.

Warden for non boss, tough, huge-damage mob

The Ender Dragon; for a non-boss mob, it's the Phantom.

Matter of context:
Looks coolest would be the dragon
Most iconic the creeper
Most useful (very tough call and even more context dependant) the guardian [​hostile] villager [​passive]

the sniffer

Mine, (of all things, you should know) Allay!