If you are going to start a thread like this, please have put some thought into the OP. It would be better if you said "Who do you think is the best wallbreaker. I think X, Y, Z is for (insert reasonably lengthy reason here), but I was wondering on everyone elses opinions. Just asking a question which is so subjective without actually contributing anything to it is pointless imo. The fact that you are asking others to give evidence without even putting forward your own argument is actually kind of lazy and a tad hypocritical. Now to add positively to this thread, I will write about my favourite two wallbreakers.
DD Salamence w/ Life Orb -
Yeah this is meant to be a lategame sweeper, but I prefer to use it early game to open up massive holes in the opposing team for something like SD Lucario to sweep. The fact that every pokemon in the game is 2HKOed by this means that even if you have to sacrifice it, it may open up the other team to a threat that they previously had covered.
CB Tyranitar -
An oldie but a goodie, I prefer to use it with Aqua Tail over Earthquake. This means that pretty much every pokemon barring Registeel is 2HKOed by one of its moves, including former counters such as Hippowdon. Choice Banded Pursuit is great for mindgames, it can either nail Blissey or it can "scare" it into staying in only to be hit by a Crunch. This is great at opening up sweeps for my special sweepers, and its boosted defense and great coverage makes it one of the best early game offensive weapons imo.