Probably above Lester is Scherzer, but I don't see either of them living up to the contract they'll get, and they're both probably out of the price range. Someone who I think does have a strong case for such a contract, and won't cost as much, is Shields. He'd be a significant upgrade on practically everyone on the Jays staff right now.
I also like Harang, and really like McCarthy, both of whom should at least be substantially better than Happ, and probably over Buerhle, who I don't see breaking much higher than 1.5-2 wins next year. Now is it a smart investment to put that much money into a pitcher of that mould when you do have passable SP and glaring holes elsewhere on the diamond? That's another question. But yes, there are guys that would significantly improve what we've got right now, and at least provide some injury insurance, something that we probably should consider, after how lucky the rotation was last year.
Outside of the conversation of strictly SP's, I think you have to at least call Villanueva in regards to a reunion. He's always been a very decent long relief/swingman type and he'd be one of my preferences for the role.