I don't know of any mini's that are completely disease free but in my garden Kristin, Glowing Amber (and all of it's lineage), Incogneto and the miniflora Whirlaway stay fairly clean all year.

I agree with Ken. As far as I know minis/minifloras are not very disease resistant as a rule. It might have something to do with their size or their proximity to the soil. I have and have had lots of them and for the most part they all get black spot like crazy. Although I have to say that I don't really see much if any powdery mildew on them. We don't really get rust here much so I can't say about that. The best one for me is Softee but it does get some spot on the bottom.

I live south of SF. The biggest problem I have in my garden is powdery mildew and Whimpsy minis are my star mini roses. I have four of them planting under my tree roses. They are pretty, smell nice, bloom all the time and rarely have any disease problems. I have two Ruby Ruby too but Whimpsy minis are so much better.

I also have two Diamond Eyes and they are healthy but they don't bloom as much.