>Charizard <--- Charizard's mega forms are amazing. Charizard-X loses his double Stealth Rock weakness, gets a kickass STAB combo, and gets a reason to use his Dragon Dance sets. Charizard-Y gets Drought and stupid Special Attack, and a bit of Special Defense to boot. Regular Charizard just plain sucks.
>Venusaur <--- Honestly, Mega Venusaur is... Meh. He's a different kind of Pokemon than regular Venusaur, being a tank rather than a Chlorophyll sweeper. The problem is his typing is crappy for a tank. Regular Venusaur might pair well with Charizard-Y.
>Tyranitar <--- Mega Tyranitar looks so good on paper, but I have yet to see him used effectively and I have yet to use him myself. Also, a lot of the Pokemon you've picked out don't appreciate Sandstorm damage. I'd just ignore this one.
>Gengar <---Mega Gengar is just broken. A Sub+3 attacks set is amazing, and you've got your choice of Shadow Ball, Focus Blast, Sludge Bomb, Thunderbolt and Giga Drain to pick from. Regular SubDisable Gengar is just as good as always, accomplishing what Mega Gengar can't always do by basically hard countering Mega Kangaskan.
>Pikachu <--- You're joking, right?
>Snorlax <--- Snorlax is a meh Pokemon all around in my opinion. He serves as a decent Special wall but with very little reliability and very little offensive presence outside of some mono-attacking Curse/ResTalk set (which Gengar eats for breakfast).
>Jolteon <--- Jolteon is a great scout and a decent offensive pivot, but my biggest issue with him is that Thundurus-T exists. While he's not availible yet, when he is he just flat out outclasses Jolteon in my opinion. Only advantages Jolteon has are that it doesn't have to run a Scarf and it's not as weak to Stealth Rock.
>Aggron <--- Aggron just has too many flaws in both of its formes to be an effective Pokemon. Mega Aggron becomes practically indestructible on the physical side (avoiding a 2HKO from Terrakion's Close Combat IIRC) but he's got absolutely no recovery outside of Rest and his offensive presence is meh since he loses STAB recoiless Head Smash. Regular Aggron is a great Rock Polish sweeper but he's got too many weaknesses (including double weaknesses to Earthquake, Close Combat, Mach Punch, etc.) and not enough raw Attack to be effective.
>Arcanine <---Infernape.
>Aegislash <--- Aegislash is an overrated Pokemon in my opinion. That doesn't mean he isn't good, that just means I don't think he's an Uber level threat. I hear mixed sets are nice, something like Shadow Sneak, Sword's Dance, Sacred Sword, and Shadow Ball or Hidden Power (Ice).
>Blaziken <--- Blaziken is a godly Pokemon to the point where it was banned to Ubers last generation (and actually got decent usage in Ubers), Mega Blaziken is just Blaziken on crack.
>Carracosta <--- Wut?
>Rhydon <--- See Carracosta.
>Dragonite <--- To be honest, I don't even think Dragonite is worth your time anymore. A gimmicky Weakness Policy set might be fun but not all that practical. He's still a decent parashuffler but parashuffling was hit pretty hard this generation.
>Umbreon <--- Sylveon > Umbreon in basically every way.
>Clawitzer <--- Never used it, never seen it used effectively, no comment.
>Yveltal <--- Overrated and just not on par with the rest of Ubers.
>Mewtwo <--- Mewtwo is just one of those Pokemon with such absurd stats that it's hard NOT to use him effectively. I'd recommend a Mewtwo-Y Calm Mind sweeper. get a single Calm Mind down and you can ravage entire teams with him.
