Well, LX and LH offer non-stops, which would be as good choices as any if you can get them on UA miles. The LH flight from MUC is the only overnight flight from Europe to DXB (besides TK) which might be an advantage or a disadvantage for you. (Similary, the flight back MUC-DXB is the only day flight, whereas the LH FRA route and LX ZRH route are overnight, leaving around the same time as UA.) I take LH all the time, and it's solid -- the angled-flat seats are not great, and entertainment choices limited, but food and service are pretty good; I prefer the A340 over the B747. I haven't taken the LX flight in C recently, but it gets good reviews.

OS is another connecting option, albeit with aircraft and service that are inferior in every way to LX and LH in my opinion.

Turkish offers decent service DXB-IST (make sure that you get a widebody), but you'd suffer the poor European-style business class on the other segments on either TK or LH/LX. I also wouldn't recommend MS.