Oh yeah, guitar amp mics.
I have to agree with e609 with a ribbon. The e609 on its own is a harsh mic for sure (so much so I sold it). On a clean, dark amp, it can really help cut through a mix if that's what you want. And adding a ribbon and blending after EQ'ing, it really gave a 3D sound. I would only use it on dark speakers.
For most rhythm guitar, SM57 is my favorite. It's crunchy, can be aggressive or laid back, not too bright or dark, can get enough low end to sound huge. And it's very easy to EQ, unlike the e609. Usually I would put on iso headphones, have the guitarist play until I found a spot where the mic sounded "brightest", and there was the tone. My favorite for most speakers, especially V30's and T75's.
I've used MD421's and didn't like them as much as 57's. It's more of a "scooped" tone to my ears, meaning it doesn't have that midrange presence peak of the SM57. It gets a lot more low end thump than the 57 does for sure. I would use it on a midrangey speaker like a V30 if I wanted a more scooped sound but I *HATE* it on T75's. It accentuates everything I already dislike about that speaker.
Oddly enough, I've never tracked
my M201 on a cab. I think I tried it once, but didn't like it as much as the SM57. It has less low end thump and a brighter sound with less of a presence peak. I think I remember it being a little grainy as well. Might be good on a little Champ or some kind of small speaker going for a vintage sound.
Two more sweet mics are the Nady ribbon and the e604 drum mic. The ribbon looks like an R-121 knockoff, and I'm sure it sounds nowhere near as good, but it's a nice warm/dark sound that is great a few feet back from the amp. Would do a cool room sound for sure. The e604 is a tom mic, but it sounds like a much smoother version of the e609...a lot like an SM57 without the presence peak. Sounded awesome for solos on brighter speaker cabs.