I haven't had many IBs...
Signatory UCF Caol Ila 1991 (age? I think 11yo)
Signatory UCF Highland Park 20yo (bottled for Binny's, forgot year)
Whisky Galore Glen Garioch 15yo (year?)
Cadenhead Port Ellen 1981/1992 (150th Anniversary bottling, FoaF'ed 2004)
The Glen Garioch was wonderful on the nose and palate, but it quickly gave me a headache every time I drank it, and I don't usually get headaches from whisky.
The Highland Park was very interesting - I've had the 12 and 18 OBs, and the Sig bottling had, in addition to the heathery Highland Park-ness, an extra bit of fire.
I'd recommend the Caol Ila to anyone who likes peat, and the PE also, if you can find one. The PE was the only cask-strength IB I've tried so far. It definitely needed some water, being at a mouth- and brain-smackingly high ABV - I think it was around 64%. Once some water was added, it was magnificent. Think of it as whisky concentrate.