I was wondering, out of the SF units that we know of, which one can we claim is the best?

For the USA I can name two:

Delta Force and SEAL team 6 (DEVGRU )

SAS for the UK.

Spetznaz (Alpha or Vympel teams) for Russia

Shayetet 13 for Israel

I've been reading incredible things about DEVGRU and Deltaforce. Also downright scary things for Spetznaz training. And a prevailing opinion online that SAS is still the best (not sure why, apart from the success rate of applicants - only 2% make it). However, DEVGRU are chosen from the other SEAL teams who are already elite, and Delta Force are chosen from the Green Berets and Rangers. Spetznaz have to endure inhumane training which is outlawed in the USA. They have to learn to like pain. Don't know much about the Israelis but seeing as how effective Mossad is, I cannot dismiss them.