Depends on what elements you want to emphasize . . .
It really depends on what gives you the biggest kicks. I've bought all the Spycraft products so far and have been very pleased. I'm just starting to play the game, so here's my thoughts on product usefulness.
First off, the core book is wonderful. It really does have all you need to have a blast with the spy genre. The other books simply provide further crunchy bits.
The Soldier/Wheelman Class Guide lays out some great prestige classes for narrowly specializing these two classes. For example, there are classes for Big Game Hunter types, SEAL's, Heavy Weapons Specialists, Heavy Vehicle Specialists, Flying Aces, Bikers, etc. It also adds to the chase rules, gives expanded vehicle stats, and covers really big guns (artillery and such). We haven't done much with these rules yet (still low level), but I could see a GM using these bits to make a nasty mastermind, equip his fortress, and run more interesting chases.
The Modern Arms Guide lays out optional rules for combat and lots more firearms stats with new special qualities that can really distinguish different weapons. Melee weapons, armor, and other equipment are also featured (there's also a free download of equipment that did not make it into the book on AEG's Spycraft site). Lastly, it contains the supercool Triggerman prestige class for John Woo-style gun fu. I'm not a gun bunny who can comment on accuracy of stats but we've already used this book a lot and everyone seemed to enjoy the new options.
Hopefully that helps.