I have not tried the Electo Harmonix, though, I have not heard good things about them.
I've tried KenRad VT231's, RCA red-base, and CBS-Hytrons. Each sound quite different, and I'd recommend trying several of these versions to discover the flavor of sound you enjoy the most. Either way, they will be a huge improvement over the stock tubes (I only use the stock tubes to burn in interconnects).
Also, if you have the DJH version, you'll want to roll the rectifier tube. EZ81's are the way to go.
I realize this is not a lot of help, but it's the most honest response; there is no 'best' tube for this preamp. Instead, it's a matter of what your personal tastes are, and what tube is the best vehicle to deliver that sound.