Honestly I have to say that after finishing Season 4 it's sky-rocketed to the very top of my list...it's so freakin' powerful, especially the finale where...
[spoiler] Rita is dead and the baby is 'born in blood'. [/spoiler]
It's one of the most shocking things I've seen on TV. The only thing better than Dexter is Game of Thrones. The Walking Dead isn't on my list anymore - no room for it, as with Rome, Mad Men, etc.
Dexter is really underrated; its sad that more people don't want it.