Anything that is cut smaller than a trunk style is perfect. Trunks will do in a pinch, but briefs/low-rise briefs/bikinis/jockstraps are much sexier in my opinion.
Boxer briefs immediately are a big turn off, and boxers even more so. I won't kick anyone out of bed for their choice of underwear if I really am into them, but it can put a small damper on the mood for me. Especially if it is our first time playing around and I just discovered your abhorrent taste in undies (if you are sporting some boxers or cheap, tacky boxer briefs I mean)...
My personal fave is a pair of well-fitting briefs in some color other than white; even better if they have stripes or a pattern or are in an unusually intense color (I am thinking of the polar opposite of dingy, faded, old, washed a million times briefs in black, grey, or navy blue that are common even among us G-boys). I know that is precisely what I will be wearing; likely some bright or vivid shade of blue/red/green/purple/etc or maybe a striped/patterned pair or possibly some with a fun print.
It is important to feel good when you don your undies first thing in the morning, so I say to those tops out there inquiring about this that y'all should wear what makes you feel good. Just keep it not white, not cut all boxer-briefy or boxery, and make sure dinginess is at a minimum and they fit right and you're set.
