Focus Blast Pidgeot: It's already easy enough as it is to compare Mega Pidgeot to Tornadus-T, but now it's even more apparent, but that's a good thing, since Mega Pidgeot inherits some of Tornadus-T's pros. What can hold Tornadus-T back is how inaccurate Hurricane and Focus Blast were, but Mega Pidgeot would never have that problem thanks to No Guard. Even though Tornadus doesn't cost the team its Mega Evolution and the ability to hold an item, never worrying about an attack missing is one of the greatest feelings in a battle. Mega Pidgeot already had this advantage, but now that it has Focus Blast to actually threaten things like Rock-types and Heatran, I can see a definite niche here. I can see this one winning for sure.
Contrary Pangoro: This is kind of a weird idea, but it has some promise. With moves like Superpower and Hammer Arm to boost his stats, as well as perfect neutral coverage with Knock Off and Gunk Shot backing them up, Pangoro could easily become a huge threat if given the chance. The problem, however, is getting the momentum to begin with, and while he has decent bulk, he has easily exploitable weaknesses in Flying- and Fairy-type moves. Pangoro also has terrible Speed, so Choice Scarf users would easily get the jump on him. But if you can get it to work, you could easily clean house late game. Also giving Intimidate, Defog, and Sticky Web users the bird would be hilarious. If Mega Pidgeot doesn't win, then I can definitely see this one winning.
Dragon Dance Arcanine: This one's fairly interesting. Intimidate can easily create the opportunity to set up a Dragon Dance, and with Close Combat, Crunch, Extreme Speed, and Wild Charge as coverage options, it can be damn hard to wall Arcanine. Plus, being a Fire-type means immunity to burns, so you can set up on Will-O-Wisp users fairly easily. I see the comparisons to Mega Charizard X, and I can see why people would choose him over Arcanine, since he has better overall stats and Tough Claws. However, Arcanine make up for those with better coverage moves, Extreme Speed, and Intimidate to better wall physical attackers while setting up, not to mention not costing the team its Mega Slot. Not my favorite, but I wouldn't call it bad.
Volt Switch Reuniclus: I can see some value in this one. With Regenerator giving Reuniclus back some health and such low Speed allowing your teammate to switch in without having to take a hit, this would be extremely annoying to deal with. This, combined with a respectable base 125 Special Attack, means that Reuniclus is going to maintain offensive momentum, and powerful momentum, too. Sadly, no STAB on it mans that it won't be THAT powerful, but Volt Switch is meant to keep offensive momentum, so that's a moot point. A good Theorymon overall, but the first two choices are very tempting.