I wouldn't say it's most improved, but I'm going to give an honorable mention to Lapras. Picking up Freeze Dry has been a supreme blessing for it. I think, with Freeze Dry and Hydro Pump, it can hit all Pokemon for at least neutral damage in the game. It makes for a great Bulky Water counter as, even with a Quiet nature, it'll outrun most Water-typed walls and hit them super-effectively. Plus, there usually isn't much they can do in return do to pretty decent bulk and Water Absorb. It's knocked out several Azumarill trying to switch into it to Superpower it because, even with a -Speed nature and no speed investment, it outruns neutral nature Azumarill with no speed investment by 2 points (unless you want HP Fire, but it still wins by 1 point).
The set I've been running is:
Lapras @ Expert Belt
Quiet Nature
252HP/252Sp.Atk/6Atk
Freeze Dry
Hydro Pump
Psychic/Hidden Power (Fire)
Ice Shard
Like I said, Freeze Dry and Hydro Pump grant it perfect neutral coverage. Ice Shard gives it priority to finish off weakened things, especially the annoying Dragonite and Gliscor that think they can get you first. The last move comes down to preference (or what you can get through breeding). Psychic grants you a super-effective hit on Fighting types, though you probably won't appreciate a Mach Punch coming off of most of them. Hidden Power Fire is for Scizor, Forretress, and Ferrothorn.
Also, if you want more guaranteed power, you can run Life Orb instead of Expert Belt, though the recoil isn't always appreciated as it'll be hitting a lot of things for Super Effective damage anyway. Leftovers or an Assault Vest can be used if you want more survivability, but the drop in power is very noticeable.
The set is rather tricky to use, but it does make a surprisingly effective Wall Smasher.