blimp:
Variety is the spice of life but "bottom" is a trigger word for many people. It is certainly my favourite word. Arse is too crude, fanny, butt, ass, tush, tail etc far too American. (Not that that is a crime of course.) Posterior sounds like an architectural feature and rump sounds like something you might buy at the butchers. Derriere is not actually an English word at all but a French one. You could try and get away with gluteus maximus but only if you are compiling a medical dictionary. I think when all is said and done you will find you are left with bum, behind, backside, buttocks and bottom (of course).
Not too difficult to come up with a few more, I reckon: hindquarters, rear, rear end, cheeks (or bottom-cheeks), rearward curves, mounds, cushions, sit-spots, target area, hinderparts, booty, seat.... There's also 'haunches', but to me that sounds too much like a cut of meat. 'Arse' can work fine in context, I feel: 'a good arse-warming' has a likeable ring to it. I tend to avoid 'buttocks' - it's such a square, angular word. (Its exact French equivalent, 'fesses', is so much more evocative; you can almost see the quivering in those double s's.) 'Derriere' is originally French, true, but I think it can count as a valid adoption; it's widely enough understood.
As GG rightly says, 'fanny' is anatomically ambiguous, depending which side of the Atlantic you're on. The title of Cleland's classic 1748 erotic novel, 'Fanny Hill' (which includes a fine spanking scene), is a naughty pun on 'mons veneris', the pubic mound.