Mega Ampharos or Mega Venasaur.
I'm doing this type-wise, you have a Fire, Psychic, Dragon, Flying, Dark, Fighting, Water and Ice.
Ampharos being a dual Dragon-type when it Mega Evolves really doesn't mean anything since it doesn't learn many Dragon moves to my knowledge, and plus Ampharos has only three weaknesses, which is good. Mega Ampharos has a ridiculous Sp. Atk stat, which is great for moves like Thunderbolt, Signal Beam and the ever so powerful THUNDER. >:) It also has a good Attack stat, not beastly, but good. (95)
Venasaur has great stats all round when it Mega Evolves, all base stats being =<100, which is great. Also, it only has two weaknesses since Thick Fat halves two of it's weaknesses, Ice and Fire. By looking at this, Venasaur looks like the better option.
Another plus to Venasaur is that it's Mega Stone is available earlier in the game, as well as your free Bulbasaur. Whilst Mareep is only available in hordes, which as you know, are at a lot of a lower level when you get to them.
So yeah, Mega Venasaur wins.
Hope I helped. :)
Source: Multiple parts of Serebii, and this.