Ki-Adi-Mundi's lightsabre (the one depicted in the AOTC Visual Dictionary) is really cool. It's sleek and elegant, plus the cylindrical guard reminds me of the Daywalker sword that Blade uses.
Ven Zallow's lightsabre is also pretty cool-looking.
And while we've never had a "proper" depiction of it I've always liked how Callista's lightsabre is described, with sea creatures carved into the hilt. Plus it has a topaz blade which is awesome.