Please read the reviews of tube types and amplifier types. These are all real reviews submitted by guitar players and audiophiles. Remember, tone isn't a science and "one man's treasure is another man's garbage".
If you'd like a recommendation for your amp, please indicate the amp brand and model, the brand and number of tube you'd like to replace, and what aspects of the amp you'd like to change or improve. Also, list your tube budget so I have an idea of how to help you. Without this information it's very difficult to make a recommendation.
All advice is based on personal experience, and/or the experience gained through other customers. There is no guarantee that you'll love anything that anyone else likes. The best way to find out what YOU like is by trying a variety of tubes.
If you need more than one 12AX7 tube and aren't sure which one to select, try at least 2 different brands/types. Typically, there are one or two 12AX7 sockets in every amplifier which are most critical and will shape the tone of your amp. These are usually the sockets furthest from your power tubes. In Fender and Marshall amps, these sockets are known as V1 and V2. See the Fender Amp preamp layout guide for more information.