Another vote for Exapno Mapcase; my library used to have Harpo Marx’s autobiography and I think my husband and I were the only people ever to check it out. We probably read it about five times each. It must have been discarded, which distresses me as we surely would have bought it.
niblethead - cracks me up.
Shirley Ujest
Freudian Slit
Vinyl Turnip
olivesmarch4th
Samprio
Enola Strait
Czarcasm
Purd Werfect
psychogumby
Already In Use
Coldfire
Knowed Out
Lute Skywatcher
Bosda Di’Chi of Tricor. It’s so enigmatic.
This was also my favorite - half because it’s a great name, the other half because she got it (IIRC) by asking her nephew what his 3 favorite things were. He reminds me of my kids 
And when he uses the sig line, “A giraffe’s tongue is 12 inches long and prehensile”, it conjures up an even funnier image.
Another vote for Don’t poke the bear, Dad!. Always at the top of my list.
Flatterer.
Stealth Potato
nth-ing Czarcasm.
I forget how he parsed it, but two and a half inches of fun was a great username. Also DerTrihs brought out the geek in me when I finally thought I had realized what it meant (turns out I wasn’t actually quite geeky enough to get it entirely).
I’m insanely jealous of Mangetout, for grabbing that username long before I ever could. It’s brilliant.
I also get a giggle out of cometothedarksidewehavecookies, Always Brings Pie and Stealth Potato’s usernames.
Hmmm… apparently I have a thing for food-related handles. 
You got there before I could.
Czarcasm is one that always puts a smilies on my face.
ComeToTheDarkSideWeHaveCookies would have to be number one.
I imagine it as a Darth Vader type voice saying Come To The Dark Side then a little kid piping up with We Have Cookies! while waving a cookie around.
pbbth
Have to make the sound effect while reading that one 
Mahna Mahna *do do do do do *
Qagdop the Mercotan always retains my attention.
I like that one, too, and I was surprised to find out where it came from: a really great, unfortunately short-running cartoon called Invader Zim. The title character, Zim (an inept alien invader disguised as an elementary school student), says it to Tak, the hideous new girl (that’s the title of the episode). Found a vid of it, the line’s about 45 seconds in: “Now, prepare your brain, filthy beast of meat and hair!”
Another vote for Cometothedarksidewehavecookies
There was also a Freudian Push-Up Bra gracing the boards for a while.
Of the other names that come to my mind, the only one I haven’t seen yet in this thread is vivalostwages.
My favorite is my own, of course :).
I also like fireunderpantsboobs and Go You Big Red Fire Engine. Are they both gone now?