My current faves are:
Mind Flayer Assassins (Dark Slayers) use their improved levitation feat after plane shifting into the realm they are working in to travel slowly towards their target without ever touching the ground, in absolute, magical silence. Once they strike their target, they then absorb the foe into the shadow-stuff they brought with them, making it 50% likely that even true resurrection, miracle and wish will fail to bring them back from the dead.
(From the Dark Slayer class I wrote for Unveiled Masters, the Essential Guide to Mind Flayers)
And my Aranea Dark Stalker Assassin. Aranea can choose one humanoid race that they can shift into. Choosing Dark Stalker grants them sneak-attack damage and major dark action bonuses that combine nicely with their natural spellcasting abilities and that set them up perfectly to become assassins.