I HATE fighting against Munna. It's so f*cking annoying. But yes, it is probably the worst in the entire game.
If we aren't going with how difficult it is to obtain the 'mon, because you can eventually just find it, I really think Scizor needs to be in the High tier. He hits just so freaking hard, and has great mobility. If it's the "Metal Coat is a pain to get", doing the main story line and getting perfect link with Eeveelution, I got the wandering salesman twice, and by the second time it came around I was able to buy almost 2 of every evolution item he had.