I love the really dark DF violets and YF/GF for body color, I like the really bold colors like Cobalt GF and really light colors like Dilutes, Mallorn is one of my favorite color combinations as the opaline YF II which makes her pale seafoam green. If I ever bred her (which I won't) I would breed her with a Cobalt Cinnamon YF II cock and hopefully get a Cinnamon Opaline YFII
Spangle opalines are gorgeous, I agree with all that's said! Here's a few examples of budgies that have some of my favorite mutations: (photos courtesy of Google Images)
Look at this stunning blue series fallow!
And this pair is gorgeous!
Found one! A Sky Blue, YF II, Cinnamon dilute
I love rainbows and TCBs, too, and anything similarly complicated.
Oh, and here's a picture of Mallorn, couldn't post one about genetic preference without including her!
There's just so many beautiful mutations, these were the ones I thought of but there are millions more. Sorry this is so long, but just had to share!