THE best exemplar is the one that looks best to you. The hand changed over the years (almost everything I know about its development I've learned from Ken's posts here), and people add their own little flourishes and adjustments, presumably because those adjustments look good and work for them. As you look for an exemplar among the excellent suggestions here, you decide what looks good, what you want to emulate, what you'd rather change,* and, perhaps importantly, what you feel comfortable doing. If your exemplar just isn't congenial to you, practice will be an exercise in frustration.
It's like my parents-in-law said when my sister asked them what was the best wine when we all went tasting at a vineyard: The best wine is the one you like to drink.
*Leave this for later until you get a better eye for what you like
I myself started learning from Lupfer's exemplars on Iampeth, which really are superb. However, many people here also like Eleanor Winters' Mastering Copperplate Calligraphy. I myself don't like it too terribly much, but of course, that's for you to decide.