Shure SM57 every time, I prefer it to the SM58 for my voice, and it is renowned as a guitar mic! It can also handle much higher SPLs (volume) than many condensers which is great for getting those tubes saturated!

If you have a slightly higher budget the Shure KSM (27 / 32 / 44) series are also very nice but require Phantom power. The KSM27 gives a lot of bang for your buck.