I like larger odd sized banners. I'm not fond of the 468 x whatevers. Busty Cafe has a few beatiful banners at 534x220 that are commanding in size, yet not so big that they mess with your layout. The banner on top of
this page is an example. I also like 350 x 200+. For me, anything that is less than an HPA, but larger than the norm works well.