With a very unimpressive BST of 455, Hitmontop doesn't look at first to be amounting to much. Fortunately, he received a couple good priority moves like Mach Punch and Sucker Punch, as well as the mighty Close Combat. But the heart of Hitmontop's power lies in one of the best moves in Doubles and one of the best abilities in Doubles; in having both, to be specific, a quality only shared by one other Pokemon yet to be released.
Hitmontop -> Double Battles
->Recycle<-
Hitmontop @ Eject Button
ADAMANT
+Intimidate+
6 HP / 252 Atk / 252 Spe
-Fake Out
-Close Combat
-Sucker Punch
-Wide Guard / Feint / Helping Hand / Detect
The namesake of this set lies in Hitmontop's item; he comes in, gets hit, and is out again, able to once more use his Fake Out and Intimidate. Close Combat is self-explanatory, and is the general choice on this type of Hitmontop, since he's going to switch frequently anyway; Stone Edge is chosen over Rock Slide both because Hitmontop is fairly slow, and because, without a huge Atk stat either, he won't be KOing what he wants with Rock Slide.
Eject Button serves in other situations as well. Say you're running a rain team yet the sun is up, and Politoed is on the field right now. You'll have to spend two turns to renew the rain by switching him out and in, right? Well, let's also say there's an enemy Exeggutor looking to use a powerful Grass-type move on Politoed. What do you do? Switch Politoed out and put Hitmontop in; he'll get hit, be ejected, and in can come Politoed, all in one turn.
The EVs are a little awkward and subject to whoever uses him; it's tempting to buff up his HP since his Def and SpD are so good, but what he suffers in Spe will mean he may get hit by an attack BEFORE he gets to attack and then ejected, wasting a move.
->Priority<-
Hitmontop @ Life Orb / Fighting Gem / Dark Gem
ADAMANT
+Technician+
252 HP / 252 Atk / 6 Def
-Fake Out
-Mach Punch / Low Sweep
-Sucker Punch
-Wide Guard / Feint / Detect
A more offensive Hitmontop, truly worthy of the term "Technician" because he is, in a sense, a specialist. He works out specific problems--a fast sweeper, a Protecting low-HP foe, an enemy that needs some stalling--then returns to the back rows, if you want him to come back later for another Fake Out. Many pokemon--especially frail Sashers--can be taken out by a combo Fake Out-then-Mach Punch or Sucker Punch. Between the latter two Hitmontop has almost perfect coverage, and covers all the Pokemon in the niche he's meant to fill. His total priority also allows him to bolster up his HP to take advantage of his good Def and great SpD.
Hitmontop -> Double Battles
->Recycle<-

Hitmontop @ Eject Button
ADAMANT
+Intimidate+
6 HP / 252 Atk / 252 Spe
-Fake Out
-Close Combat
-Sucker Punch
-Wide Guard / Feint / Helping Hand / Detect
The namesake of this set lies in Hitmontop's item; he comes in, gets hit, and is out again, able to once more use his Fake Out and Intimidate. Close Combat is self-explanatory, and is the general choice on this type of Hitmontop, since he's going to switch frequently anyway; Stone Edge is chosen over Rock Slide both because Hitmontop is fairly slow, and because, without a huge Atk stat either, he won't be KOing what he wants with Rock Slide.
Eject Button serves in other situations as well. Say you're running a rain team yet the sun is up, and Politoed is on the field right now. You'll have to spend two turns to renew the rain by switching him out and in, right? Well, let's also say there's an enemy Exeggutor looking to use a powerful Grass-type move on Politoed. What do you do? Switch Politoed out and put Hitmontop in; he'll get hit, be ejected, and in can come Politoed, all in one turn.
The EVs are a little awkward and subject to whoever uses him; it's tempting to buff up his HP since his Def and SpD are so good, but what he suffers in Spe will mean he may get hit by an attack BEFORE he gets to attack and then ejected, wasting a move.
->Priority<-

Hitmontop @ Life Orb / Fighting Gem / Dark Gem
ADAMANT
+Technician+
252 HP / 252 Atk / 6 Def
-Fake Out
-Mach Punch / Low Sweep
-Sucker Punch
-Wide Guard / Feint / Detect
A more offensive Hitmontop, truly worthy of the term "Technician" because he is, in a sense, a specialist. He works out specific problems--a fast sweeper, a Protecting low-HP foe, an enemy that needs some stalling--then returns to the back rows, if you want him to come back later for another Fake Out. Many pokemon--especially frail Sashers--can be taken out by a combo Fake Out-then-Mach Punch or Sucker Punch. Between the latter two Hitmontop has almost perfect coverage, and covers all the Pokemon in the niche he's meant to fill. His total priority also allows him to bolster up his HP to take advantage of his good Def and great SpD.