Kabutops is definitely the most useful of the fossil Pokemon, as it can actually hurt things. Its typing is awesome as it resists Normal attacks and Grass is reasonably rare—yet it does survive an Exeggutor's Mega Drain! Its Ground and Electric weaknesses do hurt it a lot, but it is definitely a strong UU option that could be successful in an OU team.
Kabutops needs a free turn to be effective, but basically it attempts to Swords Dance and sweep. If your opponent has a Starmie that doesn't have a status effect, then you may need to Body Slam for paralysis, but once you get past Starmie, Kabutops can do all sorts of damage. It switches into Chansey's Ice Beam nicely or, of course, any Hyper Beam, preferably from a paralyzed Tauros or something slower than it. It destroys Persian, Surf Snorlax, or Dodrio, and of course is a great switch-in against a sleeping Pokemon.
Slash is just an awesome move, but unfortunately it will ignore your Swords Dance Attack boosts. Blizzard is for Grass-types, though this is only useful in UU.
Checks and Counters
Gengar takes little from Surf and lashes back with Thunderbolt; Starmie works similarly, except it fears boosted Normal attacks a bit more. Electric-types don't enjoy Hyper Beams, but they also strike Kabutops with Thunderbolt. Venusaur and Victreebel slam its quadruple weakness to Grass with Razor Leaf. Exeggutor with Mega Drain may work as well.
In the UU environment, Kabutops can be potentially dangerous. Tangela and Magneton are the two best counters in this metagame. Tangela has the ability to replenish HP with 4x super effective Mega Drains, while Magneton can paralyze Kabutops and hit hard with Thunderbolts. Wigglytuff can also fare well with its ability to use Thunder Wave and Thunderbolt, but it just isn't as reliable as Magneton.