Any 616 Ultron, as far as we've seen, would have a hard time against Thanos. The only Ultron that could take the titan would be the buffed up version that took on Kang.
Ultron has casually wrecked an Avengers team consisting of several heavy hitters though, the Age of Ultron version that was punking Thor would put up a fight.
A good version of classic Ultron would put up a fight, but I don't see that he could put Thanos down, at least not for the count. Thanos would probably resort to some sort of tech.
Not really sure about Phalanx Ultron and what he was capable of in a one-on-one fight.
The future Bendis Ultron would win.