I don't play MB often, but its stage 5 background is probably the most screwy one I've seen, especially during Megumi's nonspells.
From the games I actually play, LLS stage 4 irritates me at the start when the bg randomly chamges while two fairy statues are firing bullets at me (bad design desicion IMO). The end of SA stage 6, as Kaze said, is probably the most solid contender for Windows. IN stage 5 around Tewi also used to distract me, but now it's not really a problem anymore.
I suppose MS stage 4 blue-on-blue could be a bit distracting as well, but I actually hardly mind it at all. :V