The latest version is actually 0.10.2 (0.10.3 if you go through the nightly releases)
The most useful for Mari0 mods would be probably 0.9.2 or 0.10.2
The one used in most Mari0 mods (and the original game itself) is 0.8.0
For your own projects, probably 0.10.2 would be the most useful
I could go on and on because of how vague the post was but I'm not that bored.