Overall, I definitely believe that it's the most effective striking art. That said, in MMA competition, there's a lot more going on than just pure striking. In that venue, a lot of different striking arts can be effective in many different ways. In a pure striking match, though, Muay Thai is great. (And no, I'm not a nuthugger; I don't even train Muay Thai myself.)
It should also be noted that while I think a paragon of Muay Thai would defeat a paragon of any other striking art in a striking contest, few people are paragons of anything and it almost always comes down to the better fighter.