I don't want to get in a fight over this, but I've just read a TON of bullshit.
1. No one on here has met a delta force team. Besides the fact that no "delta force" is actually confirmed by the US government, any team resembling them (best of the army special forces) is kept away from civillians during training because of the classified nature of their operations.
2. Marines aren't outright better than the others. That's a load of crap that they sell to get recruits. Read any training manual, bio, etc. It's all the same superior dogma. I've known several marines (and seals) and I would not even begin to consider this. Marine force recon is a different story. The reasons why Marines have such a good battle record are: A. They are sent in much larger teams. B. The jobs they get aren't as tough as SOF's.
If I had to rank them though, I would say that the true TOUGHEST are:
1. SAS.
2. SEALS.
3. Army SF
4. Marine Force Recon close behind.
Now keep in mind that I'm not saying that one is better than the other, but the training of SAS and Seals (and FN if they were in this) is tougher. Skillwise, you would have to include ST6 and their army counterpart ("delta force") because frankly, the average SAS soldier blows the others out of the water.