Haven't played many of these, but I'll comment on a couple...
Royal Troon is a beast when you play in into the wind. It just seems like it never ends, never lets up. Going out with the wind is no treat either, trying to control the ball.
Carnoustie as the toughest? Not in my experience at least. First time I played it in a 25 mph wind, and while I had major duck hook-itis on the front leading to a 49, I got myself straightened out on the 10th tee, and after nearly killing a caddie in the group ahead went on to a 38, including -2 on the final three which none of the caddies in my group had ever seen done before. The second time I visited I went around in 83 in 18 holes of pouring rain. I've never played in rain like that before, I'm mostly a fair weather golfer.
After that second round at Carnoustie I played four more rounds in Scotland, shooting 76, 102, 76, 82. Turnberry, Prestwick, North Berwick, Muirfield. Guess which one I'd vote for as the toughest? I've done better at Prestwick though, the first time I played it I went round in a mere 95! OK conditions were tough both times, but from what I hear from the people at the club it is pretty much always that way. It isn't like the same strong winds affected me at all at Turnberry the day before.
I'm sure I could break 90 at Prestwick if only I had a gallery to find my balls for me, it isn't like the rough is that thick there. If it had the hay that Muirfield does I'd probably be a pile of bleached bones lost somewhere off the 5th fairway
