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While Hydreigon has a really cool design, partly being based off King Ghidorah, its special attack stat of Hydreigon isn't enough - it has a rather subpar Speed stat, and a crippling weakness to Fairy - Primarina pretty much stops it. However, if you know how to use it, it can be devastating - I've successfully been able to clean up teams with it in late ORAS.
As for me, I'd have to go with Garchomp. Not only has it got a really awesome design (it's a land shark!) it also has a devastating 130 attack stat, it also has good Speed to boot as well. Scarf sets can really weak havoc late-game.
I certainly haven't used all of them recently enough to do a great comparison, but I've always found Tyranitar to be an amazing physical attacker. I might have to go back through Sinnoh and try out Garchomp since I haven't seriously used it in around a decade.
Garchomp is the one I'd vote for, if only because it's the one I've had the most negative experiences with facing in battle...and the most positive experiences with using. I'm not much of a competitive battler any more - to the contrary, all my knowledge on the subject is dated to the end of the fourth gen, haha - but even in the anime Garchomp is typically portrayed as all-powerful, whilst the others, not quite so much. Except for perhaps Salamence, given how vicious J's was.