The core set is the best box so far. Everyone needs at least one, I recommend two (I have four, but that's another story).
One of each is a good recommendation. Plus a second set of Saboteurs, if you want to play skirmish.
If you want to Play mercs in skirmish, the Elite Tuskens (1 unit per Twin Shadows) and the Regular Trandoshans (1 unit per core box) are great.
If you want to play Imperials, 2 core boxes are minimum, and a second Hoth set is very good, too.
Edited by DerBaer