In terms of HTC phones, I've had the Kaiser and Raphael before the HD2, and I must say that the call quality on the HD2 is the best of the three! This wold be both through the speaker on the back (speaker phone) and through the earpiece.
I agree that the initial volume was way too loud, however there is a tweak somewhere on the board where you edit a registry entry for initial call volume, and that helped tremendously. So I'm actually quite pleased.
Kaiser (HTC Stock) - Retired
Touch Pro (Voda UK) - Retired
Leo HD2 (HTC Stock) - To the wife
HTC Inspire 4G (AT&T, Rooted) - Retired
Samsung Galaxy SIII (AT&T)