I've used stock Mesa 12AX7s, Electro-Harmonix 12AX7, Groove Tubes 12AX7-C, JJ's ECC83, and Tung Sol reissue 12AX7.
At the moment my amp is configured with the Tung Sols in V1 and V2, high gain JJ ECC83s in V3 and V4, and a balanced ECC83 in V5.
The JJ's ECC83s have a very dark tone. When I had them in V1 and V2 they absolutely sucked the life out of my clean channel.
The Groove Tube 12AX7-C is Chinese made. It has a much brighter sound than the JJ's. You can expect very dynamic cleans and detailed harmonics.
Currently, the Tung Sol 12AX7 reissue is my tube of choice for the V1 and V2 positions. They are a Russian-made tube, but not nearly as "dark" sounding as the JJ's. It is a very well-rounded tube. I get pristine, 3-dimensional clean tones and excellent crunch and distortion.