Here are a couple from my collection that I find pleasing, probably because they are neat--well placed stamps and cancels, no smudging, etc.
For some reason, the plainness of this cover appeals to me. I couldn't bring myself to soak the stamps off this one. (Addresses have been digitally obscured.)

This one has a nice postage meter and attractive corner card. I also like the typewritten AIRMAIL in red.

Regards, Robert