iTunes link: Marvel Comics - Free
It's an obvious choice, but one worth highlighting. Marvel's comic book application owes most of its success to the publisher behind it, we'll concede, but the presentation is pretty good, too.
The vibrant colour reproduction of the iPad's display is also important. You can't get comics looking like this on a Kindle. The only limitation is that you'll not find a single Batman comic available to read, and that's a crying shame.
The clever sliding panel transmission effect is nice, but it's the archive of comics available that makes the Marvel app shine. The collector in us still isn't ready to give up on physical comics yet, but this is a compelling alternative.
If you're a Marvel Comics fan be warned: you'll almost certainly end up investing a scary amount of money into building a massive digital library of titles. But it'll be worth it.