Yeah, in my opinion a powerhouse is a pretty strong, usually evolved pokémon (Pikachu is more of a super-powered glass cannon), capable of easily obliterating an entire team of an inferior trainer's pokémon, but also able to put up a tough fight in a league / gym battle (particularly league).
Examples would be Paul's Drapion, Electivire and Torterra. Magmortar and Ursaring too, although they haven't been shown to be especially good. That guy from the Silver Conference's Blaziken was definitely a powerhouse too - it even took down Charizard (albeit sustaining serious, almost-fatal injuries in the process). Drake of the Orange Islands' Dragonite would be another. As would Lance's Dragonite and Gyarados.
From Ash's pokémon, I would say Infernape, Charizard, Sceptile, Snorlax, Torterra maybe (although it's also defeated rather easily, I'd say that's partial lack of compatibility with Ash, who works best with fast, powerful pokémon), and Heracross. I could see Tepig eventually becoming a powerhouse (although I'd rather it stayed unevolved), and possibly Snivy or even Oshawott.
Obviously, Infernape, Charizard and Sceptile are the defining powerhouses. Sceptile was uber-strong, easily overcoming type disadvantages against May's Blaziken, and defeating a Darkrai (Which was weakened a little by others), even snapping out of Dark Void-induced Sleep. Infernape, with its' Blaze, is incredible, particularly when you consider what it did to Paul's Electivire after taking all that damage. Charizard is another obvious powerhouse, defeating legendaries and just about everything else it's ever faced, except for a Charmander (because he effectively forfeited) and whatever froze his wings. Charizard, like Sceptile, has overcome type advantages, when battling against Blastoise.
So yeah, that's who I consider powerhouses to be. And I have just realised how huge this post is xD