Damn you...making me choose between Goshujin-sama and Onii-chan. It was a tough choice but I have to go with Onii-chan, it just seems cuter and more of a turn on to me.
I can imagine "Onii-chan" getting annoying over an extended period of time, so I would rather "Master". Or we could be honest and say I have a huge maid fetish.
I did pick Onii-chan, but reflecting on it I think I'd rather have 'Master' instead. Though I'd have electrodes implanted in her erogenous zones and a Man-mote control for said electrodes.
Goshuujin-Sama hands down for me. Onii-chan does have a nice ring to it, but most of my loved characters fit the "Goshuujin-Sama" more so than Onii-chan.
Personally, being called "Master" is one of the things that really turns me on. So I have to go with Master. XD I mean having the obedience of a girl and her being devoted to pleasing you HAS to be a big turn on.