Hi,
Being a guitar player not happy with 57, i will suggest following,
The royer 121 is very good-agreed.
But don't forget:
Sennheiser MD-421-never bad on a good sounding amp.
Beyer m-160- also good
Coles 4038 ribbons, it's figure of eight so it has that cool room color.
MD 441 is also nice sometimes.
Theres plenty of mics that could be cool on guitar, it's to a degree just about wich color youre after. Most of these mics have a pretty "smooth" sound. And they all have some kind of mid-range peak and mostly dips a bit to somewhere else in the midrange, as do the sm57, its just in different places, and thus might be good depending on the exact application.
High level amps tend to be very snappy and responsive, dynamic, and detailed so smoother sounding less detailed mics might be ion order as above.
But, for low SPL amp operation a condenser might be nice, bringing out more of whats there, but lots of amps are relatively noisy on low levels, and a condenser won't make that less apparent...
AKG C535 is a pretty cheap condenser, unlikely choice, but for some reason is really nice on twins, or similar amps.
U 87s can be nice on a low level amp, but needs a few inches of distance.
tough the last time i recorded a very low level amp, i was a micstand short, and so i just hung the 87 by the cable over the tilted twin, worked very nicely.
On VOX ac30 kind of amps, some tube condensers can be killer. But it depends a bit on what kind of role you want the guitar to play in the mix.
also of course, at least one room mic, wich might be very different characterwise than the main mic, might bring a lot of life to your guitar track.
These days i'm really into a combo of beyer m-160 as main and the dynamic omni Sennheiser MD-21 as room.
Hope this is helpful-
-bora