i'd say it depends on your budget. I've used a few, but only used GVO and Aweber seriously (e.g more than 1000 subs) and Aweber wins by a mile!
GVO is ok as a starter but it lacks important tools such as the ability to segment your list, and doesn't have very good reporting.
With Aweber you can segment, use automation (e.g being able to unsub somebody from 1 list automatically when they subscribe to another), and do other things that make business easier.
So for me Aweber is fine. I've heard good things about GetResponse too.
Main negative for aweber is the cost, it increases dramatically once you get a few 1000 on your list, I think I pay about $150 at the moment which will increase some more once I pass another couple K subs.