So I am a tad bit biased.
Taco del Sol is home to Mission style burritos, tacos, nachos, Jarritos soda, cheap beer and wine margaritas, rotating art for sale, and smiling hipster counter staff. Although the locations off Broadway or Reserve are nice if you want to get in and out, the original downtown location offers better people watching, a better vibe, better tunes, and a much higher likelihood of running into someone you know, even if you are just passing through Missoula.
Personally, I am a fan of the Mission with chicken and black beans and guacamole, but other friends are fish taco and veggie burrito devotees. The half size is still huge.
Consistently voted by the local alternative weekly as the best place to eat in Missoula for under 7 bucks and the best place to eat alone. I couldn't agree more.