Many will leap in and say a good ol' SM57, and it works for lots of people. Personally, I've had my best luck with the following- Sennheiser e609, Sennheiser MD421, Shure SM7b, AKG C2000B. The first three are all standards. The last one, C2000B, is the mic that no one but me (as far as I know) uses for cabs, but with about $12,000 bucks worth of mics in the house, that's the mic that sees the most duty on cabs. No one has figured it out, but in blind test after blind test, the C2000B has beaten out a whole lot of mics that cost a lot more, are intended for cabs, and which are considered studio standards for that application. I have no real explanation, but I can tell you it's a great cab mic. It's a very versatile entry level condenser that I used to use for lots of stuff. As my mic cabinet has grown, it has been replaced for almost everything except cabs and hand pecussion/rack toms, where it is still my #1 choice. And it's not just me. In blind tests, a wide variety of guitarists have preferred electric guitar tracks done with C2000B over a ton of other mics. It still surprises me.-Richie
P.S.- oops, I see you have an SM57. I think SM7 or MD421 will do what the SM57 does- better. C2000B does something that none of the above do. If you are looking for something different, do it. You can get them bew on ebay for $150 or so. Look for one with the H100 shock mount, which is one of the most versatile shock mounts out there.