My two favorite villains has (of course!) been the two no party has yet managed to slay! Nothing gets up a players nose more than an enemy that keeps getting away from them.
One was a half-fiend troll with a handfull of fighter levels and a gem with a permanent anti-magic field on it sown into his chest cavity (no-one's figured it out yet!). Because of the half-fiend template the troll had great physical stats, INT 10 and acid, cold, fire and eletric resistance 20.
It's unbelievably hard to kill a troll with a high fire and acid resistance within an anti-magic field!
The other one - my absolute #1 favorite - isn't a typical evil villain really. He's just incredibly annoying!

His name's Albright Eberhart Jr. He's geeky, bookish, spoiled, grudge-bearing, bragging, cowardly and a daddy's boy. He collects books, bugs and books about bugs. A nuisance, nothing more... If only it wasn't for all those wizard-levels!
When the PCs first stepped on his toes -yes, THEY started it!- he sailed as a shipmage on one of his daddy's ships, supposedly protecting it from magic-using pirates. When the pirates actually did strike, he kept busy protecting his own behind for a few rounds, before teleporting to the safety of daddy's mansion!
The PCs hated him from the minute they saw him. Something about him seems to send out a message to the world, saying "I'm utterly useless, a waste of space and air, but if you're not nice to me I'll tell daddy!"
'Daddy' was himself an adventurer in his youth, one of the worlds most famous spell-inventors, before he retired and invested his accumulated loot in a sizeable mansion and a small fleet of cargoships and fishing boats. He's still an honorary board-member of the local arcane order, though.
In his cabin (the door was marked "Albright Eberhart Jr. -
MAGE ") the PCs found his extencive collection of dead bugs -one of which was a stuffed giant bee, a large trunk full of clothes -all of premium quality, very neatly folded and with little tags saying 'Albright' in lovely needlework inside, another trunk holding all his books ('The Big Book of Bugs', 'Giant Ants or Formians - What is the Difference?' etc.) - all of wich had printed on the first page "Albright Eberhart Jr. -
MAGE", except for one: 'The Beginner's Guide to the Planes' had the inscription "Albright Eberhart Jr. -
PLANESWALKER". The bed was made up without a single crease. A small water elemental was trapped in a barrel in the corner. On the wall was a framed
Scroll of Limited Wish; 'To Albright, from mom and dad' was printed underneath it. There was also a framed drawing of a girl (medium pretty), with a lock of brown hair tied up all neat and pretty with a red ribbon. There was a toad in a spotlessly clean goldfish-bowl; immediately recognized as his familiar and stamped on repeatedly by the players - they hated the guy that much already! Last of all they found a well-hidden, beautifully giftwrapped book, labeled "To my little butterfly, with love from Albright'. (It was one of those magic tomes that up your charisma, can't remember what they're called right now.)
The players never really hit it off with poor Albright...

Since that first run-in an intens, mutual hatred has built up between them. They've tried to kill him, he's hired others to try and kill them. But young Albright has proved hard to get rid of. He's rich and he's got access to all sorts of magic goodies through his beloved daddy and his order. He prefers spells and items that protect himself and always has a few
Dispel Magic ready to counter the spells of others wizards. All these defensive spells means he hasn't got many slots for the offensive ones, so once he's blasted of those, he teleports away - usually with plenty of hit-points still left and leaving my players increasingly more frustrated!
If only my players could get at his spellbook... Then they'd realize that he's actually four levels below them. Now, that would
really bug them! And of course, sooner or later Albright Eberhart Sr. will be after them... That should put the fear of mages in them!
