Unconditionally, I've found that Cyscarf (Choice Scarf) is incredibly durable, and with a timid nature, this Poke' can outrun quite a few Pokemon. It can't outrun Salamence though, who still manages to switch in on Cyclohm and absolutely destroy him.
I use him as my lead, mind you.
Cyclohm@Choice Scarf
Shield Dust
EVs: 100 Def/158 Spe/252 SpA
Timid Nature
Draco Meter
Hidden Power Grass
Thunderbolt
Surf
Incredible type coverage. Hidden Power Grass can 2HKO Mamoswine, who is without a doubt Cyclohm's biggest threat. That's just a precaution however, and I've actually never died to a Mamoswine lead. Draco Meter is for all leads who aren't in some way effected by the other base moves--a Choice Scarf'd 140 special damage, stab dragon move 2HKOs almost every Pokemon and will always OHKO a dragon (unless said dragon is Salamence who in a one-on-one confrontation is always faster). Thunderbolt is self-explanatory, and surf is for pesky Heatran and other grounds.
The three counters that always come in and give me grief are Dugtrio, Latias, and Salamence.