Slowbro as DA said is an excellent Fighting type counter, as it can take anything most Fighting types throw at it.
However, Slowbro does have the problem of being weak to Bug, which means Heracross can cause it a lot of grief with Megahorn. And that can be a problem, as Heracross (not Hitmontop, not Mienshao, and not Scrafty) is the most popular Pokemon in UU. For better walling against Heracross, Cofagrigus is a great choice. It has excellent defenses and can easily take a neutral hit or a Night Slash. And if that Heracross is Burned and increasing its attack through Guts, well Mummy takes care of that. It also proves helpfull against Technitop. Another problem with Slowbro is weakness to Grass. And the introduction of Virizion to the tier can mean trouble.
Walling Fighting types is hard. Mienshao doesn't stay in long, Heracross has Megahorn, Virizion is just plain awesome (and has Grass typing), and Scrafty does a good job of screwing both Slowbro and Cofagrigus. So you might be best off destroying them. Tornadus is a new member of UU and was only thrown out of OU because of its Therian form. Torndus makes short work of all Fighting types. Just be careful about the switch, you might end up eating a Stone Edge.