Well, you have to see "best piece of the set" from two perspectives:
1. Mini on its own:I would definitely go with Han GH or General Dodonna.
For example, look at Luke Force Spirit! On his own everyone would say "he can move 4 squares more than Obi, what a great mini", but on second glance he lacks the synergy with his faction, that Obi has with Rebels (Leia of Cloud City, anyone?).
2. Synergy with the rest of the faction:There are so many pieces that really rock, it is hard to tell.
Dodonna works with any squad, but since Rebels are mostly about Cunning Attack, his usefulness is limited to a certain degree (though still he is very powerful, no doubt).
Tyranus is a new beatstick for the Seps that in contrast to most others is VERY playable. He adds a lot to the Seps' faction.
Nyna is very powerful, too. She has tons of synergy with the Imps, so she is very near to the top of the set.
Han Solo, GH. Great piece, with solid stats and awesome abilities. A little fragile for his points, but he is a shooter and that makes up for it.
I could go on with this list for quite a while since Legacy was a great set. I see no mini that towers over the rest since almost every faction in the set got something useful. Only the Sith still have too few medium cost minis (20-35 points) and lack synergy.
Right now my personal vote for "best" mini of the set would go for Han. Being able to negate init control and still being very playable makes him what Ford made Han in the movies: UBERCOOL! His abilities also indirectly help the Jedi in the NR, so he is a great gun and "leader".
But as I said, that is only my personal preference and only by a thin line. At my LGS we play era tournaments at the moment, which leads to interesting squads. It also gives me the chance to play a lot of minis without focusing too much on the same old squad (unfortunately I can play Black Sun anytime and it is so hard to let go of them Assassins...
). Thanks to Legacy there is something useful and fun for any era. Great job WotC!