At one time or another I've ran all the saws listed by the OP except for the 6421. So my comments are based on direct experience not regurgitating something I read someplace.
Echo hit the 60cc market pretty hard, CS-590, CS-600P and CS-620. Leaving my brand loyalty behind and the fact that I work on power equipment for a living my vote for the best 60cc offering currently available is the CS-620PW. It may not be the most powerful as I've not timed tested it against any of the competition, but power to weight is excellent, ergonomics, smooth/broad/flat power curve, runs longer bars with good authority (tested it clear to 30"), fuel efficient and to date 100 percent dead solid reliable. I'd also add that the CS-620PW is very easy to work on, and one can remove the carburetor in less time than it took to type this.
For the average guy who needs a dead solid reliable 60cc offering and under $400 to spend get a CS-590. It sports many of the features of the Pro versions and a very well made saw. Just remove the limiters on the mixture screws, grind off the tabs, put them back on and give it some fuel. I like to remove some of the deflector on the muffler as well. Nothing out there in that CC range will cut as well for the money.
The "brand loyalty" thing will never let any Echo saw join the ranks with the commercial industry so don't expect to see them on too many job sites. I've introduced quite a few to folks in this area who do tree removal and/or cut a lot of tops, firewood, outdoor boilers for heating, etc, and to date they absolutely LOVE the CS-590 as a daily firewood saw.
One of my customers uses one as a ground saw for his crew, over 2 years now getting totally abused on a daily basis and working just fine.
Stihl and Husqvarna offer excellent 60cc Pro saws so they have the market covered pretty well and their offerings are good enough to keep out intruders. As for the "king", it will be one of those and not an Echo or any other "off brands".....IMHO......Cliff