Which EL34 is your favorite and why?

I second the Gold Lion tubes.  They give you that rich midrange.  

For currently manufactured tubes my preference is Tung-Sol followed by JJ. My favorite supplier for these is Tube Depot. I have purchased many preamp, driver and power tubes from them over the years and not once had a dud.

Ditto Jond's recommendation for Gold Lion KT77's.

+2 @jond, we have been selling quite a few of these of late, including a set to pdreher for his Music Reference RM-9. For me, the Siemens EL-34s have been the best I have experienced in Music Reference, Cary, and VAC amplifiers.

Disclaimer: I run Tube Audio Store.