Depends on how loud you play live actually. The old workhorse SM57 is usually the "go to" choice for micing loud live amps as it handles high sound pressures very well. I have seen some using condenser mics as well for live amps, but they were more of a blues/jazz type of gig. The SM57 or SM58 are almost always the choice for loud rock.

cgtrox

Are you talking about play live, or in the studio?

If you're talking live, either the SM57 or e906 would be a great choice.  You can hang the e906 from the cab directly over the speaker.  But, be careful because it's easy to get extra noise from the mic rubbing on the speaker grill cloth.

If you're in the studio, then try as many mics as you can that'll handle higher SPL's.  You can find the specs for these mics on their makers' websites.  I'd say they'd need a rating of around 120dB or so.  However, after trying all these, you may still end up on the SM57.  There's a reason that it's one of the most commonly used mics for guitar amps, whether live or studio. 

We could all talk about the details of mic'ing here for days.  But, if I were locked away in a room full of guitar amps with only one mic, it'd be an SM57.