Been a lurker on this site for awhile but this definition of 'ace' finally motivated me to make an account. I have come up with a somewhat interesting way to define an "ace". It has to do with what I call a lockdown start. I define this as
5-6 IP: 0 ER, 7 IP: 0-1 ER, or 8-9 IP: 0-2 ER
If a pitcher has such a start in at least 25% of starts I consider that an 'ace' year. If a pitcher has done this in at least 2 out of their last 3 years I would call them an 'ace' pitcher.
With that your 2021 aces are: (AL) Ray, Cole, Rodon, Bassitt, Eovaldi. (NL) Wheeler, Scherzer, Urias, Buehler, Miley, Burnes, Woodruff, Gausman, Wainwright, Alcantara, Desclafani,
Some notable exceptions: Peralta, Lynn, Ohtani, Kershaw (not enough IP) Musgrove, Stroman, Mccullers, Berrios (not enough lockdown starts)
One might ask, how is Eovaldi with his 3.58 ERA an ace but Stroman with his 2.88 ERA is not? Well that is where some other stats come in handy. Eovaldi leads the AL with a 2.72 FIP. Stroman has a 3.27.
I went with lockdown start as my metric because it evokes what a lot of us think of when we picture an 'ace'. Has a good chance to completely shutdown the opposing lineup everytime they take the mound. I quite literally have only had this idea in my head for the last 2 hours though so I'm sure there are holes and bad outliers, would love to hear further discussion.