For GUI clients, Thunderbird is the best I've seen, followed by Evolution (back when I used it, anyway).
Though if you're open to a ncurses-based client, I use mutt. The #1 reason I switched to it (instead of any other client) when the new Gmail design was released was because it opens almost instantaneously. It's a subtle thing, but not having to leave my inbox open all the time and being able to open it in a split second when I need to really helps my workflow.
I probably going to use newest Thunderbird when I leave gmail.