Swampert was the best Tank of the Gen IV era by far.
While Swampert's movepool upgrades from the previous generation are not numerous, they are very significant, and make Swampert into the top-tier Pokemon he is today. Swampert is one of the most solid defensive Pokemon in the OU tier, as he is capable of effectively setting up Stealth Rock, attacking, and defending. His good defensive typing leaves him with only one weakness—Grass-type moves—that can be easily played around through proper team support. While powerful special attacks may hurt Swampert somewhat, he can take most neutral hits with aplomb, and his resistance to Rock- and Fire-type attacks, as well as immunity to Electric-type attacks, grants him plenty of opportunities to switch in. Overall, Swampert is a great choice for any OU team, and is a defensive threat to prepare for.
For more information on Movesets and EVs, check here.