This link has some nicely compiled information from WashU regarding the match statistics for various specialties. Of course, the information presented is limited to the raw NRMP match data and is not informative as to the relative quality of applicants applying within any given field (ie, it does not account for self-selection). However, based purely on these numbers an approximate hierarchy of relative competitiveness across fields in 2005 would read as follows:
1. Plastic surgery (43% match) >>
2. Dermatology (57%) >>
3. Rad Onc (72%) >
4. Radiology (78%) = Ortho (78%) = Uro (77%) = ER (79%) = ENT (77%) >>
5. Ophtho (88%) = Neurosurg (88%) = Path (88%) = Surg (88%) = Anesthesia (90%) = Psych (91%) >
6. Ob/Gyn (95%) = IM (98%) = FP (98%) = Peds (97%)
Again, this is only a rough categorization based on rather crude statistics, however, with some notable exceptions (for example optho and psych are not realistically in the same tier of competitiveness as far as I can tell) it comes relatively close to most people's estimate of reality.
Interestingly, based on the median salary divided by average weekly work hours posted on the site, the hierarchy would run approximately as follows:
1. Ortho
2. Path
3. Rads
4. Derm = Ophtho
5. Uro = Plastics = Neurosurg = Gen surg
6. Gas = ObGyn = ENT
7. ER
8. Psych
9. IM = FP
10. Peds
Of course, these are only mean figures and don't account for subspecialities (for example interventional cardiologists in the IM category) as well as malpractice costs. Hopefully, there's more behind one's choice of residency than mere dollars and cents, but I think that the breakdown is informative nonetheless.