I’ve heard that if you go outside the city the mixian gets better and better. Is it true?

Depends on where you go.

san francisco

You got to be kidding. The best mixian is the one you make yourself. You know what's in it. First it was yogurt and now is mixian. What's next, who has the best baozi?

@Liumingke: Maybe, but it's got a lot to do with personal competence - mixian may not be what you'd call haute cuisine, but in this department I have little competence, and I'm sure there are good professionals out there who know plenty of things that I don't.
This is also the case with hummus (Syrians especially), kuskus (from the Maghreb), and barbecue (North Carolina and Texas, but I've enjoyed some of the very different sorts one finds in late-night market areas of Kunming).
P.S.: Of course there's always lousy mixian to be found made by, um, less-professional professionals.

This is a good one for douhua mixian.

三碗豆花米线

m.dianping.com/shop/98047454

Jianxin Garden (建新园) is a good one.

Jianxin Garden is an old restaurant which has a long history, its Guo Qiao Mi Xian (Bridge-crossing rice noodle) taste fresh and unique. And its Cuiwang rice noodle, assorted cold rice noodle is recommended.

docsbay.net/famous-special-restaurants-in-kunming

► Specialty: Guo Qiao Mi Xian (bridge-crossing noodle), cool Guo Qiao Mi Xian
► Consumption per person: 20 RMB
► Address: No.13 front Jinbi Street, Jinbi road, Xishan District, Kunming
► How to get to Jianxin Garden: taking bus No.3, 4, 62, 73, 90, 98, 107 to Jinmafang stop

By far the best Mixian I ever had were in a place called wenshan zaodian at Xiaoximen 文山早点(小西门分店)
(j.map.baidu.com/fgFh-). You can add your favorite flavoring and then add their freshly brewed beef broth to it. And it costs less than 20 Yuan, so really a great deal.