That would either be Unearthed Arcana, or the FRCS.
Unearthed Arcana is a vast toolkit for customization of 3.x, and with it you would have the rules to run dozens of campaigns and adapt the 3.x rules to lots of styles and settings.
Forgotten Realms Campaign Setting may be a 3.0 book, without any 3.5 "crunch", but it's the single best one-book treatment I've ever seen of a campaign setting. The fact that the Realms (pre-Spellplague, which as far as I'm concerned never happened in my campaigns) are my favorite setting is a big bonus. It's my favorite setting summed up into one book.
As for which of the two? That's a really, really hard go. I guess it would depend on if I was going to be running just one game or those would be the only books I would ever have. If it was just one game, the FRCS because I could run a very fine FR campaign with just the Core 3 and that book. If it was ever, then I'd go with UA so I would have plenty of ways to change up and keep the game new over many repeat playings.