So far, Airachnid. And to those complaining about Bulkhead, I have him, and he is one of the best figures. He may look weird, but seriously, he's awesome.
The Fallen is better than anything in the Prime line. The rest of Titanium is total junk though. (I'm retracting this because I just looked up a review for The Fallen on youtube and he is nowhere near as cool as I remember.)
Jetfire's OK too. CobraTank Megatron is the worst of the bunch due to product quality issues. I think Metal is best left for toes and chest plates and not for arms. I'd be tempted to vote for Optimus Maximus or Weaponizer Bumblebee over Airachnid personally. though yes Airachnid is a pretty horrible toy.