I've been trying to work out which single-shot launcher provides best performance to LEO, by looking at the Ariane  and Boeing Delta-IV  launchers (no particular reason; these are just the launchers I thought of as being capable). I got the latest versions of the documents I referred to available, but I suspect they may be a little out of date (four years old).
Another thing to note is that the Delta-IV is only available for US government launches now, because Boeing was finding the commercial market insufficiently lucrative.
I'm interested in the maximum lift capability to LEO, pretty obviously, and for my purposes I'll consider a 45° inclination orbit at an altitude of 1000 km.
The Ariane 5 ES costs about $150,000,000 a shot (probably more). It consists of two solid fuel boosters and a cryogenic (liquid hydrogen/oxygen) main stage, with a solid fuel upper stage. Its performance to a LEO as described is just over 18,000 kg.
The heavy variant of the Delta-IV (Delta-IVH) costs in excess of $120,000,000. This is the only figure I could find, and it's even more outdated than my documentation for the launcher; I suspect the true price is closer to $200,000,000. The launcher consists of three Delta-IV CBCs (cryogenic main stages) strapped together, with a cryogenic second stage. It's a pretty big launcher. Performance to the aforementioned orbit is about 22,500 kg, in a payload bay of comparable size to the Space Shuttle's.
So for sheer mass capability, the Delta-IVH beats the most capable current member of the Ariane family hands down, and has a comparable price tag.
 Arianespace, 'Ariane 5 User's Manual', Issue 3 Revision 0, March 2000
 The Boeing Company, 'Delta IV Payload Planner's Guide', October 2000