Pokémon X and Pokémon Y

SuperNerd I'll give it a shot, though I do think there's an older thread for this kind of stuff. I like using bReakMyTeam when I make teams, just to see if I have any glaring weaknesses.

 ramble

Gengar: Not a fan of Destiny Bond on Mega-Gengar, since most things that will take you out have priority or are Scarfed. Seems like a wasted moveslot to me. Maybe grab Substitute instead, to ease your fears of fast, frail Scarfers and priority users. Dazzling Gleam and Thunderbolt are also nice choices for their respective coverage.

Clefable: Eh. If you're going to use a Fairy as a Cleric, I much prefer Sylveon or Florges. They can WishPass, but you'll either have to drop Protect, Aromatherapy/Heal Bell or Toxic.

Greninja: Not a fan of using Greninja as a scout. U-Turn's damage output is kind of pitiful. Personal opinion: I'd pick either Grass Knot or Extrasensory over U-Turn.

Talonflame: The EVs are oddly specific... Are you having lots of trouble with positive base 101s (i.e. Thundurus-T)?

I don't like using Pokemon 4x weak to Rock without a Defogger/Magic Bouncer/Rapid Spinner. Roost helps somewhat, but when you have to pick between Roost to avoid KOing yourself from recoil, Swords Dance to actually do some damage and an attack, you'll be hitting yourself for not getting rid of Stealth Rock first.

Goodra: Pretty standard fare besides Dragon Tail. If you're going to phaze someone out, you better have some Hazards up to get that residual damage going. Spikes is hard to set up and can be easily taken care of by most Defoggers, since they aren't even affected by them.

Ferrothorn: Like I said, Stealth Rock over Spikes.

Wow, amazing website. I didn't even know about it (I use mariland for that >_>).