The best one I have found so far is the M&K MX350, quicker than my MX5000, but less powerful. I found the Rel ones slow. I liked the Velodyne DD12 and DD15, they are fairly quick also.
The M&K 350 is difficult to find in decent nick, expect to pay around £350 - 500 depending on condition.
I use M&K because they are relatively quick, pretty much unbreakable, go fucking loud and deep, with quite a bit of flexibility on settings.