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My vote for most improved definitely goes to Kangaskhan. It's mega-evolution is a true beast. 2 attacks, the first at full power and the second at 50% power, but both have the full capability to inflict secondary effects. Not to mention, first hit will destroy a substitute, and second will still hit it.
1) Power-Up Punch on Mega-Kangaskhan becomes a damaging Swords Dance.
2) Sucker Punch allows it to do nasty things to Gengar and Mega-Gengar.
3) Fake Out gives it a "free hit" on just about everything.
4) Substitute needs no explanation.
5) Dizzy Punch gives it a STAB move that does decent damage, and when combined with the new mega-evo, enables it to (fairly) reliably cause confusion.
Its fairly diverse move-pool also includes:
-Earthquake
-Outrage
-Aerial Ace
-Rock Slide
-Shadow Claw
-Toxic
-Return (STAB-boosted)
-Fire/Ice/Thunder Punch (once Pokebank launches)
It also has access to a good number of SpA moves that provide coverage, such as Fire Blast, Blizzard, Surf, Thunderbolt, and Solar Beam, although its SpA is really not good enough to take advantage.
It also has a few options ability-wise:
-Scrappy allows it to hit anything that might switch in while it is boosting with Power-Up Punch (Gengar, I'm looking at you).
-Early Bird allows it to use Rest as a fairly reliable recovery move.