Hi kslampi,
There are lots of answers to this question. Microphones are like shoes, you can get a different mic for every situation and you can spend as much as you'd like on them. There aren't really hard and fast rules for "best", but there are some generally accepted guidelines. For acoustic guitar, most folks reach for small diaphragm condensers (SDC's) first, so that's probably where I'd start if I were you. As far as which one, that's really a question of personal preference and price. AKG, Audio-technica and Rode make popular inexpensive SDC's, but I'd probably look at one of the boutique mic makers/modders before going that route (Advanced Audio, Oktavamod and JJ are probably good places to start). If you have some money to spend I'd probably start by looking at Schoeps and Gefell. The Earthworks QTC30's that CJ mentioned are commonly used, but I'm not a giant fan of them...I'd use them as room mics on a choir or a string ensemble, but not much else. My pair pretty much just sits in their case. That doesn't mean they're bad, it just means I'm not really a fan of how they sound for most of my applications. Poke around the interwebs and you'll find lots of people that love them.
The other option you should consider is a pair of good general purpose large diaphragm condensers (LDC's). The thing about LDC's is that they'll generally sound pretty good on most things. The same is not generally true for SDC's. I'd feel just fine putting my U87 or Beesneez Mahalia on an acoustic guitar, a vocalist, a guitar cab or a clarinet, but I would never put my Gefells in front of a cranked Marshall or a loud rock vocalist (and never, ever my QTC's).
Hope that helps,
Dean