Of those 3, Showtime based on programming. Based on High-Def, I don't know. I've heard Showtime's high def isn't always the greatest though. I'd imagine HBO is ahead in HD.
We're into movies though which is why we went with Starz (and why I voted Showtime here) when we switched to Dish. We had a bunch of HBOs with our digital cable and when we were in college. HBO was ok, but unless you're just an avid fan of their original programming then it's not that great as it feels like that's pretty much all that's ever on leaving little in the way of movies. Their shows are top quality for sure, but there's so many strong shows on other channels now too that I just don't see the point of paying extra for pretty much more of the same (except with the option to take things to an R-rated level.) Showtime has a lot of original programming too, but not as much and it seems to have more movies.