Depends who you believe...
Kew Gardens say it's - 1) Grapes, 2) Citrus (Lumped together, I know!), 3) Apples, 4)Bananas.
Another site will tell you citrus was slightly ahead of grapes in 1999 as number one.
Elsewhere, a claim is made that it's actually Palm Oil nuts!
Alas! Tomatoes don't feature.