The 12AX7 is the most common preamp tube found in most amps, and is without a doubt, the Dino Preamp Tube of Choice.  The 12AX7 is a nine pin triode that's basically two tubes in one. Each half can be utilized separately or together. In the early days of guitar amplification there was a gain stage, a tone stack, and then a make up gain stage. A few manufacturers had two gain stages before the tonestack rather than one before and one after. Today some amps have as many as five or six gain stages. The 12AX7 has the highest gain rating of any of the nine pin preamp tubes. A rating of 100 or more. For comparison purposes some other nine pin preamp tubes are the 5751, a gain factor of 70, the 12AT7, a gain factor of 60, the 12AY7, a gain factor of 45, the 12AV7, a gain factor of 41, and the 12AU7 with a gain factor of 19. 

Of note. Many sites are telling guitarists that preamp tubes can be interchanged to achieve different levels of gain. While in a way that may be true, some of the swaps would not be truly optimized without other changes being made to the amps circuitry. The only swap I advocate is the use of a 5751 in the first gain stage of an amplifier that needs a bit of taming. My Mesa F-100 had a problem smoothing out under high gain and I used a 5751 in the first gain stage on a close friends recommendation. It was the perfect solution.