Metagross has sadly fallen a lot in Generation V, its just not very good at all any more. It also doesn't have any synergy with your team (aside from not taking Sandstorm damage, but plenty of mons do that and its not even an essential quality for Pokemon on your team) and doesn't resist your major weaknesses, fighting and water, and it adds another ground weakness to Tyranitar, Excadrill, and Lucario, and since Earthquake is one of the most common coverage moves in the game, that's pretty bad. Rotom-W could work, but note that it and Jellicent fill very similar spots on your team (bulky waters to cover resistances) with Rotom-W being immune to Ground and Jellicent being immune to Fighting. Two waters on a team isn't necessarily all that bad, but beware of the Grass weakness you're forming even if not very many Pokemon use it-Venusaur is a real threat.