I agree with Suzi – the brain! But then I’m pretty biased as I’m a neuroscientist. The brain is involved in almost everything we do from moving and breathing to thinking, remembering, all our senses… I could go on!
Having said that, you need your other organs to stay alive so I’d rather not have to choose just the one!
I agree with the others. But I can go further: there’s a story in Roald Dahl’s Tales of the Unexpected where a guy gets his brain taken out (along with an eye, connected to it), and the brain and eye survive in a special fluid to feed them, while the rest of his body dies (he has cancer). It’s pretty much fantasy, I understand! But still, it’s a nice idea – as long as you could communicate in some way. As Tim said, you’d need artificial versions of some other organs to make life tolerable really.