I'd be spoilt for choice - too many fantastic paintings in existence!
I really like 'jeune homme assis au bord de la mer' by Hippolyte Flandrin... but I already have a print of that, so I could live without the original
I'm tempted to opt for 'the raft of the Medusa' (Gericault) if someone would give me a big enough house to hang it in
On the other hand, there's one in the National Gallery of Judith beheading Holofernes, and, dammit, I can't even remember who it's by, but it's really good!
One of my all-time favourite is 'Europe after the rain II' (Max Ernst).
(Hmm, sounds like I only like older paintings, but I really love contemporary stuff too!)
But, if it really came down to it, there's a Francis Bacon painting, I think it was called 'two figures at a window' (or something like that), I saw it in a commercial gallery in London years ago and I wanted it soooo much and never saw it again. So I think that would be my final choice.