M3 and V3 are probably the most versatile, because they're the most popular. That means they have tons of clothing, morphs, characters, etc. Especially now that they are free.

David is my favorite. He morphs and poses much better than M3. I love him to pieces. But he doesn't have as much support as M3.

Ditto Steph Petite. She's my favorite female, but doesn't get as much support as V3.

Apollo looks really interesting, but I haven't bought him yet. I want to see how much support he gets first.

I don't care for James and Jessi. Their proportions are quite odd. Inhuman, even. Especially the arm proportions. They are so odd that it makes posing difficult.

Aiko and Hiro are really appealing characters, that have a lot of support. Though meant as anime figures, they have a "realistic" mode that's, well, a lot more realistic looking. They also have extremely expressive faces. (I guess it's easier to do that with toonish characters.)