Most natures you have aren't really great for the Pokemon, but considering you'd have to breed new ones to change that, I'll just list the best case scenarios. Also, rather than adding on, these are directly in opposition to most things DGatrz has said.
Hydreigon @Expert Belt
Rash Nature
-Dragon Pulse
-Superpower
-Flamethrower
-Dark Pulse
This pretty much ensures Hydreigon can hit anything and everything in the game for at least neutral damage. Hydreigon definitely doesn't want an Impish nature, as it lowers its beastly special attack. As such it's primarily geared towards being a special attacker, but with its usable attack, a surprise Superpower can deal with a lot of things that would normally counter it. Plus, the attack drop from Superpower is largely inconsequential since you can continue to spam Special Attacks after it. Expert Belt is a decent option for an item (in-game) since it complements Hydreigon's great coverage with no drawbacks.
Mienshao @Life Orb
Adamant/Jolly Nature
Regenerator
-Hi Jump Kick
-U-Turn
-Rock Slide
-Fake Out
Poison Jab is a terrible choice for any non-poison type (and even then it's still not great on poison types themselves even with STAB) due to poor coverage. Fake Out may seem unconventional for in-game, but it has the niche of breaking through Sturdy, which proves to be a fairly common annoyance in Gen 5. The rest are just there for obvious coverage, and U-Turn works well with Regenerator. Life Orb may not be popular in-game due to the recoil, but Mienshao abuses it quite well due to regenerator; when it switches out it easily recovers the lost HP.
Blastoise @Leftovers
Modest Nature
-Scald
-Ice Beam
-Toxic
-Protect
Blastoise isn't the best offensive Pokemon, with only average offenses, but stellar defenses. It doesn't make good use of a mixed set due to said offenses. Scald/Ice Beam offer great coverage with one another; Ice Beam hits the Grass and Dragon types that resist water pretty hard, and Scald's chance for a burn makes it great on more defensive Pokemon like Blastoise. Toxic and Protect make use of Blastoise's combination of bulk and leftovers, allowing it to stall out stronger opponents that you might not be able to KO outright. Waterfall/Ice Punch instead of Scald/Ice Beam can be an alternative since your Blastoise has a +Attack nature.
Beartic @Muscle Band
Adamant Nature
-Icicle Crash
-Stone Edge
-Superpower
-Hone Claws
The Ice/Rock/Fighting coverage is pretty obvious, but Hone Claws is there not only for an attack boost, but due to the fact that it helps patch up the imperfect accuracy of both Icicle Crash and Stone Edge. Muscle Band is an obvious item that boosts all your physical moves with no drawbacks; it's great in-game.
Archeops @Flying Gem
Naive Nature
-Acrobatics
-Stone Edge/Rock Slide
-U-Turn
-Earth Power
Flying Gem Acrobatics can and will 1HKO anything that doesn't resist it; even if you can't continuously replace the flying gem, you still have a 110 power STAB to spam. Stone Edge vs. Rock Slide is all about whether you prefer power or accuracy. Earth Power is there because it completely destroys things that would otherwise wall Archeops to hell and back; namely, Rock and Steel types. Given most are physically defensive, and Archeops has a very usable special attack, it can make great use of Earth Power.
Mew @Wise Glasses
Modest Nature
-Nasty Plot
-Psychic
-Shadow Ball
-Aura Sphere
Mew is one of the few Pokemon to learn Nasty Plot, and it makes fantastic use of it. It has the bulk to ensure it'll be able to use said Nasty Plot, and the Psychic/Ghost/Fighting trio of moves ensures pretty flawless coverage. Boost, and proceed to use the appropriate move on whatever comes in on you. Wise Glasses are the special equivalent of Muscle Band, and again, an obvious no-drawback choice for in-game.
