How are we doing this? Lore wise or in game battle wise? I'll do both just for shits and giggles.
In terms of megas in lore, I'd put my bet on Mega-Aggron. It has Filter along with a ridiculously high defense stat, to all attacks from Tyranitar would do very little. Plus if Tyranitar puts enough force behind its attacks, odds are it'd end up chipping itself on a lot of the spikes lining Mega-Aggron's body. Plus Mega-Aggron's plating that lines its body could potentially snap off someone of the rocks that extend from Mega T-Tar. Mega-Aggron is at a height disadvantage, but it beats out Mega T-Tar's weight by 300 pounds.
Mega T-Tar would definitely have an advantage when it comes to special attacks, but Filter still reduces the power of stuff like Flamethrower by 25%
In game it's a similar situation with Aggron being stronger in a 1v1 versus T-Tar. Here are the calcs.
252 Atk Tyranitar-Mega Earthquake vs. 252 HP / 4 Def Filter Aggron-Mega: 93-111 (27 - 32.2%) -- guaranteed 4HKO
0 Atk Aggron-Mega Heavy Slam (40 BP) vs. 0 HP / 0 Def Tyranitar-Mega: 84-98 (24.6 - 28.7%) -- 98.6% chance to 4HKO
So yeah T-Tar loses Mega vs Mega.
Alright time for non-Mega.
Aggron loses Filter, gains back its Rock type which now means a 4x weakness to Ground, loses its plating, has less spikes, it's not looking good for our friend here. It is now taller than T-Tar, as well as still heavier, but yikes man. A whole ass Earthquake with those weaknesses? Not gonna be good for him. Aggron is very territorial, which may make him angrier and thus stronger if he fought T-Tar in his home turf, but T-Tar is just Pokemon godzilla in terms of strength. This motherfucker rampages through mountains and villages for fun. Aggron uh, Aggron replants destroyed mountains to make his home prettier. Very cute, but not all that intimidating.
In-game, it's a toss up. But Imma give it T-Tar here, odds are even with sturdy it'll fuck up Aggron, mainly because it could just hold a Focus Sash and outspeed next turn.
