Plastic surgery - probably the most competitive right now
Cardiothoracic/Cardiovascular
Vascular
Laparoscopic
Trauma/Critical Care
Colorectal
Thoracic
Hand (well, kind of. it's really an ortho/plastics specialty but it exists)
Transplant (kidney, liver, pancreas)
Hepatobiliary
Pediatric surgery - competitive becasue there are VERY few spots
Surgical Oncology
When it comes right down to it, if you finish a general surgery residency and did ok on inservice exams and have good letters of rec, you can match into most any of these. Pediatrics has historically been tough only because of the limited spots. Although many say plastics is tough to match (and it is right now), a lot of that stems from the fact that many people are looking at the plastics lifestyle. There are still a lot of plastics programs out there. Laparoscopic can also be difficult simple due to the limited spots.
Another thing is that just because you can do a fellowship in something doesn't necessarily means its accredited or boarded. It does however open up many opportunities.