Romulus, with time to prepare.
But seriously, I think Sran
is probably right. A sensible Romulan Empire could probably unite its disparate factions and rally against a war-weary Federation, if not to victory at least to a stalemate. The Romulans have always demonstrated an ability to find a way to not lose, even if they don't win.
On the other hand, who says the Romulans would be sensible? And, who says the Federation would be war-weary as much as battle-hardened.