My Swinub has a Naughty Nature and Larvitar would have a Brave/Adamant Nature.
My team is Ampharos, Gyarados, Heracross, Quilava, and Crobat.
Go with Mamoswine. It covers some weaknesses of every other member of your team with its STAB, plus it's easier to obtain than Tyranitar. You get Mamoswine just in time for Clair, while Tyranitar may take until the post game to get, what with Johto's level curve. Ice and Ground are also better types offensively than Rock and Dark.
Hope I helped!