Only had one BJ standing and I wasn't all that impressed, I know the girl usually gave great head as it was about visit number three so putting it down to not being comfy stood up.
Sitting is a favourite as long as the chair/sofa is comfy. Good access for the WG to nibble your nuts when sitting too.
Laid down comes a close second but fails on less convenient bollock access than sitting although its best if you like her sat on your face while she's at it
My most memorable BJ was from an English lass in oxford (not worked for about 18 months). Flat was a tip and if I'd have seen her before she spotted me I'd probably have done an about face but I was a little to polite to leg it after she started waving and saying hello from her door!
In I potter, paperwork done, cloths off and onto the bed (which was in the lounge...) and she proceeded to deliver one of the best BJ's I've ever had! Cock gobbling, sloppy, bit rough, nut sucking and everything else she could possibly throw into the mix to make me blow my load!
If it hadn't been for the fact she had "on the buses" playing on the TV behind her I'd have blown my load in five minutes dead, managed to hang it out for nearly twenty before I dumped my muck in her mouth