Name: Mega Venusaur
Typing: Grass/Poison
Ability: Regenerator
Stats: 80/82/108 (+25)/130 (+30)/135 (+35)/90 (+10) BST 625 (+100)
Movepool Additions: Ancient Power
OU / Uber: OU
Justification: Now, granted, Regenerator doesn't have much flavor on it other than it being a flower, but hey, nothing that gets Regenerator makes much flavor sense either (save for Ho-Oh, Tangrowth and maaaybe Alomomola). This helps it with recovery a lot, with how unreliable Synthesis is; just switching out gives you back a chunk of your HP. This allows M-Venu to set up Growth at the right time (preferrably with Sun up, though it works without that too), and then sweep pretty well. Ancient Power was available to Venusaur back in Gen 2, and now it's here so that Talonflame can no longer switch in safely if it has taken prior damage, and if M-Venu is at +1 it's just flat out dead. It's also M-Venu's best move to hit Heatran... which isn't much, but still.
Name: Mega Charizard X
Typing: Fire/Dragon
Ability: Intimidate
Stats: 78/129 (+45)/113 (+35)/109/105 (+20)/100 BST 634 (+100)
Movepool Additions: None
OU / Uber: OU
Justification: Look at that black dragon and tell me it isn't intimidating. Intimidate helps it with Dragon Dancing, as while the loss in power hurts quite a bit, it now has a more easy time at pulling off a boost, with HP to spare for Flare Blitz. If you want, you can do a physical wall set too, as with Intimidate and 252+ Def, LO Kyurem-B can't even OHKO it... not that you would want to stay in, anyways. The spread gives it good bulk along with a decent Atk (by Mega standard), so while it might not be as powerful as the regular one, it can still excel both offensively and defensively.
Name: Mega Charizard Y
Typing: Fire/Flying
Ability: Competitive
Stats: 78/99 (+15)/88 (+10)/144 (+35)/95 (+10)/130 (+30) BST 634 (+100)
Movepool Additions: Grass Knot
OU / Uber: OU
Justification: Competitive seems like one of these generic abilities which just fits on absolutely everyone. And after calcs, It's even more of a nuke after the Competitive boost than the original one with Drought. For reference, 248 HP / 192+ SpD Heatran is now immediately murdered by Focus Blast instead of taking 57.6 - 68% damage only. Add that to the great 130 Speed, and you get yourself a sweeper whenever you switch into Intimidate, Defog and the like. Grass Knot is there to replace Drought Solar Beam, as without that Water-types become much more of a problem than usual.
Name: Mega Blastoise
Typing: Water
Ability: Analytic
Stats: 79/93 (+10)/135 (+35)/140 (+55)/125 (+20)/58 (-20) BST 630 (+100)
Movepool Additions: None
OU / Uber: OU
Justification: With that level of bulk, the massive SpA and Analytic, this allows M-Blastoise to punch holes in the opponent's team while tanking hits. Now, Water/Steel might seem fitting, but the added weakness to the really common Fighting- and Ground-type isn't exactly appreciated, so I kept it pure Water. The bulk spread makes it so half of the tier can't even 2HKO it, giving it much time to retaliate, especially with Wish support. It does lose out Mega Launcher-boosted Aura Sphere and Dark Pulse, but now its Scald, Hydro Pump and Ice Beam are a lot more powerful with its lowered Speed. It can still use Focus Blast as a substitute for Aura Sphere, but uh, that accuracy...