From those three, Dedenne is the weakest link. While it does have a base speed stat of ~105, his SpA is ~80 and every other stat is below 80.
Aurorus, while being slow, gives you an excellent coverage against both Flying, Dragon, Grass, Water (Freeze Dry is Super Effective against Water types, 4x Damage against Bulky Kingdra and Palkia), Ice, Fire, Bug and Ground. You can't go wrong with it.
Gardevoir is the only one in that list that can mega evolve, and that in itself is awesome as heck. Fairy typing covers for it's usual weakness to Dark, while adding weakness to Poison and Steel. Gardevoir's drawback is that it's moveset is very restricted, even TM wise. You won't be able to cover for it's Steel weakness at all (unless you count Focus Blast, which is too unreliable).
Personally I'd go with Aurorus. That huge amount of coverage is much more useful than the ability of going mega but having a limited movepool from which to manage.
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