Most fun to paint SM model/unit thats decent in games and not too expensive

[Tycho]

Well i rekn the most fun model would be a commander.

You can extensively convert it to make him look really cool and unique.

Chaplains and Librarians are fun to paint too and in most cases their armor wont be the usual colour of your armies.

Hope you have fun

Cheers

[NiteRabbit]

I vote for the Tactical Squad box as a) it forms the backbone of your army, b) they are rather enjoyable to build and pose, even the trimming and gluing together, and c) you can actually do some really neat conversions with them. I just made a Captain and Codicier Librarian from Tactical Marine bitz, for example (although, having said that, it obviously doesn't look as good as the new libbies that GW is releasing soon, but it's in plastic). I'm sure that, with the right mix of bitz, you could even convert a Chaplain from a tactical squad box, or Veterans, members of a Command Squad, Devastators... The possibilities are many and varied.