most difficult speaker to please

Which speakers have you found MOST difficult to drive, place etc??
I have not owned them, but I know people, that do own them....Apogee Scintilla.

Fairly large, 20hz bass, and less-than...1 ohm load....a real amp eater.

(While I do own 2 pair of Apogees...I don't plan on ever owning these).

soundlab speakers; will bring down some of the best amps available.
Along with the two above, mbl 101s, notoriously power hungry. As far as placement goes, I've been lucky, the speakers I've owned have been pretty easy to place, which is the reason I picked them, not having a dedicated listening room.
I literally saw an small Adcom 60W/Ch amp go up in smoke trying to drive a pair of the orig. M-L CLS speakers... Luckily it was the dealers misfortune and not anyone who owned it! ;-)
I saw some Infinity IRS Betas make an ARC D250 "weep and moan" (clipped like a barber)...
Im definitely gonna second Dave's Apogee Scintilla. Loved the sound..but that 1 ohm load was a killer on all but a select handful of amps that could drive them back then..
Apogee Scintilla by far. Why do you ask? You design amps?
The original Thiel CS5 was also a very difficult speaker to drive. Scintillas do seem to be the toughest, though.
Dweller,......Hmm...I currently run a set of Infinity Betas in my system and I don't think they are that difficult to drive at all. I own a set of Kappa 9 and they are MUCH more difficult to drive than the Betas.

Both the Betas and the Kappa 9 need to be biamped.

They guy I bought my Betas from was driving them nicely with two Pass Labs X-250 stereo amps. The Kappa 9 bi-amped with a pair of X-250 will send them crying for their mama.

Old tubes? I don't know...
I heard this happen at a Stereophile show in Santa Monica circa 1988 (it was a D240 BTW, sorry).
At this same show, I admonished Dan D'Agostino ("do you think those are big enough?") for displaying some rather large amps (Krell? What's a Krell?).
Live and learn...
Of those I've owned:
Ohm F
Apogee Duetta
Apogee Caliper
Bose 901 (original version, Series 1 circa 1971)
Not the most difficult load ever, but, I had a pair of Egleston Rosa's that just ate up a 600 watt Mac amp (602). It's no Apogee but this relatively small speaker had quite an apetite for power.
definately not an expert...actually know almost nothing about apogee speakers, but i seem to remember reading that if the Apogee Full Range is run direct it present a 0.1 ohm load.
Apogee Scintilla, other Apogee and my old Diapason by Shahinian Acosutics