I know Lester is the name, and he had an excellent season (2.80 FIP).
Lester's previous three seasons were 3.59, 4.11, and 3.83 FIP. So this is really the first year in four where he's been more than a #3/#2 guy.
Scherzer: 2.85, 2.74, and 3.27 FIP last three years, with K/W ratios of 3.8, 4.3, and 4.0, and massive K volumes ((231, 240, 252). Dominant stuff.
Shields: 3.59, 3.47, 3.47, 3.42 FIPs his last four seasons. Despite pitching in Kansas City, he's still given up lots of HR's; the HR's could get ugly in Wrigley field. (Did any WS announcers remind us how cavernous and HR-unfriendly KC is?)
I think Scherzer's stuff is lots better, and perhaps with a little more margin to lose a little stuff and still be excellent.
Stuff-wise it's Scherzer >> Lester >> Shields. Not really close. The dollars will set up accordingly.