Top 3 of the fastest creatures:
1) Draptor - 340
2) Quara Mantassin - speed 295
3) Midnight Panther - speed 290
As we can see, the answer is not so obvious. Underwater creatures seem to be slow, but their base speed is huge. Underwater is the slowest ground.
The slowest creatures (I do not count monsters which cannot move):
1) Moohtant Wallbreaker - speed 40
2) Slug/Ladybug - speed 52
3) Adventurer - speed 53
What is interesting, there are creatures with even less base speed, but they cannot move completely, like Spit Nettle and Minotaur Totem - speed 39.