Another vote for 3 feet! Even though sometimes it's not quite that much in my garden. Plus, depending on the plants you are growing, that space shrinks considerably by the time they mature... like corn or tomatos or squash or...
IMO it depends what they're rows between. If it is rows between a densely-planted 3' wide "wide row" or square-foot-style block, I need a wider path b/c I have to bend and reach wayyy over and need room to arrange my feet (and sometimes butt ) to do that.
Whereas if it is rows between tomatoes, I don't mind tiptoeing along a 10" wide board through the tomato jungle.
For bush beans also I only need the narrowest of paths, b/c I plant them in a 2' wide swath (3-4 plants wide) and straddle it to pick them, so I just need room for one shoe on each side of the row.
If you have a giant market-style garden and need to get a tiller or a wheelbarrow in there, obviously you need rows that are sized accordingly.