Neutral (7) is ideal for most veg though brassica's like it neutral to slightly alkaline (7-7.5).

i was going to say depends on what you want to grow. but most plants cope with around neutral as fractal says.

Most veg will grow in a range of ph levels but as above the nearer to neutral the better. Veg are a lot less choosy over their environment than flowers are. I have never even tested my allotments ph level although I have done for my garden (Where I grow only flowers)