Great question! The importance of lettering choices is so overlooked. The art can be spectacular, but if the lettering is bad I won't read it. I agree with a previous poster here that Wild Words is a great comic font. That's what it is made for. Anything similar to that should be great, too. Best rule of thumb: NEVER USE COMIC SANS. Nothing says amateur more than Comic Sans.
Something I'm also seeing a lot on webcomics: no padding on the word balloons. If you look in a real comic, the words NEVER come up close to the edge of the balloon. In manga there is often a LOT of white space between the words and the balloon edges. It's important. I'm not sure WHY, but it is all the difference in looking like professional lettering.
I use Wild Words in my series An Earth to Defend, and hand letter my other one, Interstellar Strike Force. Obviously, I'm not the best hand-letterer yet, but I'm hoping to get better as I go along.
It's sad when I see a well-drawn comic with bad lettering. (Mine included, sometimes!) Spend as much time and effort on the words as the drawing, because they are just as important!