I've bought and used ALL of them, it seems, and
ComicRack is the reader of choice in my opinion. Having said that, it's not cheap, and while it's a great
reader in its own right, where it really shines is when paired (and synced) with the
desktop application of the same name - so its full gamut of features may not be of much interest unless you're using ComicRack to manage and catalog your comic collection. For those of us that do, it is GODLY.
Prior to ComicRack's release on iOS, I used
ComicGlass. A little plain-jane in appearance compared to some of the flashier alternatives, but lacks nothing in the way of viewing options and preferences - once setup just the way you like it, it really just gets out of the way and lets the comics do the talking.