Rapidash is a very cool Pokémon, but, like most Wrap/Bind/Fire Spin users, needs a bit of luck to go along with it. Rapidash can really hurt stuff with an Agility under its belt, though.
Rapidash is quite an underrated Pokémon that can cause havoc in the UU environment, though it isn't the best UU Pokemon to use in an OU match. Agility is good if you get paralyzed, or to trap stuff that is faster than Rapidash if you can get one under your belt, and Fire Spin can be used to hit the incoming counter and then switch out safely. Whittle the opponent down with Fire Spin, then finish them off with Body Slam or STAB Fire Blast. Fire Blast has a chance to burn Golem and Rhydon on the switch, and paralyzing an incoming Starmie with Body Slam is cool too.
Horn Drill is a nice option if it's unbanned, given Rapidash's Speed. Hyper Beam can be used for offense's sake, while Stomp is a good option to flinch or paraflinch your foes.
Checks and Counters
Starmie gets special mention for its ability to outspeed Rapidash before an Agility and nail it with Surf. Slowbro doesn't take much damage from any of Rapidash's attacks and can Amnesia up in its face, while Alakazam can outspeed it and Recover when need be. Chansey can paralyze it, but an additional Agility will give it back its Speed. Faster Pokémon like Persian, Tauros, or Dodrio having moves that can severely damage Rapidash are effective counters.
In UU, Golduck and Poliwrath take little from Rapidash's attacks and can set up on it with Amnesia. Vaporeon's 130 base HP and 110 base Special laugh at Rapidash's special moves and Surf can seriously damage it.
In general, remember that, even with an Agility, Rapidash is nowhere near invincible, as Fire Spin's accuracy is an extremely shaky 70%. One or two misses will probably do Rapidash in, whatever the tier it's being used in.