12AX7 I've heard a lot of these.
These are the best in my opinion.
1. Shuguang Black 12AX7LS from Grant Fidelity. (Top grade version. Need 20 hours burn in so comments based on that. The best of both 2+3 below. Black backgrounds, great soundstage, best bass and tight through the frequency range. Tons of detail.)
2. Telefunken ECC83 (clean, neutral, transparent, detailed.)
3. GE 5751 black plate triple mica (70% of the gain but grain-free and black backgrounds. Small but very detailed soundstage)