Well that's a tough one! I only own 3 saws, and won't count my Dad's.
So, first is my 421. Bought it new, switched to the euro catless muffler, and really learned how to tune a saw on this baby. I've cut more wood, by a mile, with this saw than the other two. Dropped some giant ash, maple, black cherry, hardhack, etc with it. Far more than it's size would warrant. Light, nimble, reliable, and has done everything I could ask wearing an 18" bar.
Then came my 562. This thing has simply performed flawlessly. Mastermind mm is the only tweak. Runs
strong, and I always wear hearing protection anyway.
Finally is the big boy. Solo 681. Simply a
beast and not close to being broken in yet. Not only the strongest by a fair stretch, obviously, but some added personal value as I did the conversion from 665 myself.
A really tough call, but emotions aside I would have to go with the 562. Nimble enough for limbing, ballsy enough for almost everything else.