I have questioned Hitmontop's power recently myself. Like people have said, he makes great support with Intimidate, Fake Out, etc. then kills stuff with Gem Close Combat. Remember, though, there are 3 Psychic types in the VGC's Top 10 most used who don't even need to put ketchup on Top before eating him. Cress has an easy 2HKO and isn't hurt much by Sucker Punch, which Cress needs to be attacking for. Meta doesn't even take half from non-Gem Close Combat and OHKO's with Zen Headbutt(I think Meteor Mash also if Top is at -1). Latios can Sub around Sucker Punch then OHKO with Psychic. Way more then just those won't have real trouble with Top. The versatility is just great, though.
On another note, Hitmontop seems to be particularly good against teams staying away from popular stuff because they lack particularly solid counters.