The ECC83 (also known as the 12AX7) is the most commonly used preamp valve. They're actually, conveniently, two small valves in one glass shell, with two symmetrical sides that can be fed directly into one another (known as cascading gain stages) to create a big signal at the output for any given small input. This is fantastic for a multitude of reasons - the main ones being that they will provide a large enough output to drive the tone stack in your amp without it leeching all the power away, and the big gain factor of 100 means that cascading the stages into each other results in creamy, crunchy distortion.
JJ ECC83s are one of the industry's favourites due to the high amplification factor, the low microphony and noise to signal ratio, as well as being both durable and economical.
It is advised against placing a JJ ECC83 in a cathode follower valve position due to the vulnerability of the filament to high cathode voltages.