Castle Blackstar.. The first adventure game I ever played on the first 'computer' I ever had - An Amstrad PCW8256
Its a text adventure I bought around 1985 and I never completed it.. Ive been playing again recently via Amstrad CPC emulator and Im still stuck on it
On the Amiga....
Hmm hard one this... I wasnt really into adventure games on the Amiga.. (Too busy playing Turrican) but completed Another World and Flashback..
For the purists among you I would have to say Monkey Island series was the best though.
On the PC....
Sam and Max hit the Road. absolute classic and Ive still got it
On the Gameboy
Legend of zelda - bought it for my girlfriend for xmas then promptly stole it to play on the train to work. Took me 4 months to complete.