Mewtwo or Machamp

Well, Mewtwo has higher CP, but its quick attack is psychic, which the common mons aren't weak to. Machamp has lower CP, but double fight moves and STAB.

Base damage:
Mewtwo: 140 * 1.4 = 196
Machamp: 90 (2x) * 1.25 * 1.4 = 315

The base damage is high enough to possibly overcome the CP difference, but this overestimates for Machamp because of the extra time to make the two charged attacks, and doesn't factor energy (or damage energy) into the mix.

This is a question poke battler can answer for you. Plug both in, and see which takes down Blissey, Snorlax, etc., the fastest and with the least damage.

If you dodge charged moves, then machamp is better (although it's a good idea to have more than one). Otherwise probably mewtwo. Also, Rhydon gets shredded to pieces by aqua tail vaporeon, I wouldn't use anything else against it.

You can't say one is better than the other because both have advantages over the other. Machamp is faster and its charge moves are more suited for dodging, Mewtwo is tankier. It's honestly just a matter of taste.

That said, I value speed above all in gym battles too so Machamp it is for me.

Definitely machamp for blissey purposes

If you're talking about general purposes it's more of a wash

generally for gyms machamp is much better gym cleaner, most defenders are weak to fighting and two bar is much more suited for gyms considering wasted energy. Mewtwo shines when going against a golden berried blissey again and again.