+1 for Euwe. Karpov just plain sucks as an annotator. Tal, Fischer, Kasparov - yes! Would add Nunn, Speelman, Alekhine, Gelfand, Korchnoi.
Favorite annotator?

Tartakower. Head and shoulders above anybody, but mostly other player's games, sometimes his own.
I also agree with Tal being a great writer.
I also like Pachman, find him real easy to read.
I couldn't understand Speelman, he has a very laborious way about him
Which of the major players, past and present, are your favorite annotators (or not), and why?
Georg Marco and Carl Schlechter they co-wrote KarlsBad 1907 it's the prize book on Tournament Chess (there isn't another like it)
http://www.houseofstaunton.com/karlsbad-1907.html
Other greats Mark Taimanov, Mikhail Tal, Siegbert Tarrasch, John Nunn, Garry Kasparov, Wilhelm Steinitz, Tartakower, Geza Marcozy and Alexander Alekhine.
Which of the major players, past and present, are your favorite annotators (or not), and why?
Tal is hardly my favorite player, but I consider him the best writer of all the great players.
An important criterion for me is the willing to annotate his losses and subpar games objectively, without excuses. Euwe, Tal, Fischer, and Kasparov stand out in this regard. (No more than about 5% of the games in the Informants are annoted by the loser.) Botvinnik and Karpov are less impressive by this criterion.