All of the new Megas are good. The problem is that some of them don;t stand out enough to justify using up the Mega slot on them.
Put simply, they're all really good if you build a team around them.
Mega Sableye is a fucking monster. Unless you have exceptionally powerful attacks, Fairy-type moves, or a few other niche things that can take it on, it's untouchable. Even outside stall teams it pulls so much weight. Mons like Ferrothorn, Chansey, and Mew become complete deadweight because they can't do a single thing while Sableye's around. Plus, there's always the threat of a Calm Mind sweep if you leave the mon in.
Mega Audino is another favorite of mine, for some reason. I've never really liked base Audino, but the Mega has really grown on me. It's best described as a Mega version of Florges. It's good, but outclassed by Sylveon, Clefable, Mega Slowbro, and Mega Sableye, to an extent. It has absolutely ridiculous bulk (can survive a theoretical Mega Beedrill's Gunk Shot), and pulls double duty like Slowbro and Sableye. It's a pivoting Wishpasser before it Mega evolves, and after, it can use a surprisingly solid Calm Mind set. Comparable to CroCune, I guess.
Mega Sceptile is really good. IMO its best set is all special attacks Timid. It's faster than everything, is surprisingly powerful, and excellent against offense. It's faster than Latios BEFORE it Mega evolves, can easily KO with Dragon Pulse, can keep itself healthy with Giga Drain, and nukes stuff with Leaf Storm. With a single Spike up, it actually has very few safe switchins. When paired with other fast offensive mons that are threatened by Thundurus and/or stuff that's walled by Rotom-Wash, it can force a really nasty 50/50 that can end in a Lightningrod-boosted sweep if your opponent mispredicts.
It's walled by Heatran, but so is half of the metagame. IMO it's not worth sacrificing bulk, power, and moveslots just to 2HKO with Earthquake.
