It has been fairly industry standard to put an SM57 in front of a guitar cabinet for decades now. In the recording studio world, most guys would say that if you can't get a guitar amp to sound good with just an SM57 feeding a Neve preamp, you have issues! Raise your hand if you've ever used an SM57 on a guitar cabinet. OK, guess that's about everyone. Not that there's anything wrong with an SM57 in front of a guitar cabinet, mind you, but there is other stuff out there. How much fun have you had trying every mic in your drawer just to see what it sounds like, eh? At the risk of sounding blasphemous I must confess, I rarely use an SM57 on guitar cabinets anymore. Even in the church world where the rule of thumb has been, "when in doubt, start with a 57," I don't.
Actually I find that I usually put studio microphones on the stage. And a great guitar microphone to start with is the Studio Projects B1. The best way to describe the B1 is like a slightly lo-fi U87 with a cardioid-only polar pattern. The high frequency character is not as neutral as a U87, but it sounds really good. I have three of those microphones in my drawer. I usually end up using it on the stage on Sunday morning for church. Another fun mic choice is the Beyerdynamic M160. This microphone seems to smooth out the top end and it sounds really good. On paper, the tech specs may show that the Beyer has a gentle high frequency boost centering around 5kHz, but hearing it, you almost want to add some energy there because of how warm it is. It doesn't seem to have the bite that an SM57 has but when you listen to the difference I'm sure you'll put the Shure back in the drawer.
My favorite is my Gefell UM70. This has the old PVC M7 capsule and boy does it sound good! This microphone is super neutral and has a very clean top end. A lot of times I find the tone from the microphone adds too much color to the already colored guitar tone. When I want guitar amps to sound exactly like they would if I stuck my head in front of it, I always grab this mic. The newer Gefell UMT70s is supposed to be a newer version with a lower noise floor, and worth a try. Of course I would be remiss if I didn't mention the Royer 121. I use this mic about twice a month. If you can nail the microphone placement, you've got it made. I tend to back it off about two inches from the front of the guitar cabinet and it seems to work with almost no EQ. And unlike some ribbon microphones that have a very low output, the Royer has no trouble giving me lots of output and leaving me good headroom on my preamps.
Usually I try and pair a microphone that works with the sound of a cabinet. For example, really bright Fender or Marshall-like cabinets will usually get a slightly darker microphone. Putting a bright sounding microphone on a bright sounding cabinet will just tear my head off, but if I put something like a Royer 121 or Beyerdynamic 160 in font of it, it will have a tendency to tame the sound. The same is true with dark cabinets. With those I reach for my Gefell UM70 or the Studio Projects microphones. But sometimes doing the opposite also works. One guy I work with brings two Suhr Badger 30 cabinets with different speakers in them. I'll put my Gefell UM70 microphone in front of the bright cabinet and the Royer 121 in front of his dark cabinet. With this setup, I can EQ his signal just by moving the faders on the bright mic, dark mic channels. There are a few more microphones that deserve an honorable mention for delivering a great sound such as the Beyerdynamic M88, Sennheiser 409/906, Sennheiser MD421, the AKG C-414, and the Audio-Technica AT4040.
I never could get to liking the Sennheiser 409 or 906 that much. To me, they are a different flavor to the SM57, but the last few times I used one, I needed to do a lot of EQ. I prefer to start with a microphone that already sounds great in front of the instrument, then make small changes to the EQ so that it fits into the mix. I should mention that I'm becoming a bit biased against dynamic microphones on guitar cabinets. It seems these days only ribbon microphones or condenser microphones seem to suit me. A few I'm itching to try are the DPA 4090 and the Cloud JRS-34.
Besides the SM57, what is your go to guitar cabinet microphone?