I ****ing hate fighting Pikachu players who love to spam the heck out of his Dsmash.
As for Snake and G&W, G&W definitely has the priority advantage over MK. I usually have to use Mach Tornado and Drill Rush to break G&W's aerials, and his jumping and recovery is insane and rather awkward to follow (is it me, or is recovering in general super easy in Brawl as opposed to the past games though?).
I've just started to finally gain the advantage on Snake players, but they're still definitely tought to beat. The trick is to use MT on rushing Snakes. I've learned from my experience that the #1 problem I had with Snakes were his rushing attacks, and his tilts. Nothing quite like MTing a Snake attempting a snake dash, mortar slide or that god**** Ftilt.
I also love tossing a Snake into his own claymore mine (it happened to me last night on Battlefield). You can also MT through the Nikita missiles. One thing you definitely need to watch out for though is MTing a Snake with a grenade. I've played a few Snakes who'd take out a grenade in the middle of my MT and it'd interrupt the attack.