This could be a long list, but off the top of my head... The current top of the list is
Land Rover Discovery 3.... because who doesn't love expensive unreliability?
Range Rover L322 supercharghed, as above, faster.
VW Touareg V10 TDI.... as above, but you need a specialist rig for engine work
VW Passat B3 TDI Estate... Sentimental reasons, They've gone silly money now, sadly.
Fiat Marea 2.0 20v..... Because 5 cylinder Italian police car... Car i possibly regret not buying second most
Fiat Croma Turbo... Rarer than rocking horse poo, see B3 passat.
Fiat Tipo Sedicivalvole
Fiat Panda 4x4 Sisley
Renault Clio Williams.... My biggest car regret not getting one that was MOT'd and priced at £850 but in need of rear arches. Even spending £2000 on the body and paint i'd have likely tripled my money. Prices of them exploded about 6 months later, can't even get a bare shell for under 2k now. Useable ones start at £5500.
BMW E38 750i... Not as good as the 740, but if less is more, just think how much more more could be.... A black one looks and sounds a symphony of evil at start up.
Volvo 850 T5, manual, of any flavour.