I agree with Dagmire about how the forces work differently, so you can't really state who is the best. You probably have a better chance with quality of Training ?
I say the British SBS (Special Boat Service)
Don't get me wrong, I have huge respect for all the special forces, and other military forces, but it's got to be the SBS in my opinion.
Not a lot is known as they are very secretive, so not a lot of people know how Elite they are, or may have not even heard of them (a lot of people I talked to have never heard of them before)
They originally recruited solely from the Royal Marines, who undertake the hardest and longest NATO infantry training in the world. Most military forces will agree with that.
SAS mainly recruit from the Army. Logically thinking, a special forces unit where the soldiers where already elite infantry vs a special forces unit where the men were trained with common infantry training, I would say the soldiers who had more elite training before hand would serve as a better special forces team.
SBS recruit from army and others as well now, although I think they still recruit mainly from the Royal Marines.
Those are my reasons