If you had to choose a single light shaping tool, you might do well to pick the Westcott Black/White Parbolic Umbrella Promo Kit with 8' Stand.
First of all it's a really big, broad light source that wraps light around your subject yielding clean highlights and rich open shadows. But due to the special shape of the umbrella there's a unique mix of soft enveloping light with the kind of crisp detail you only get from reflectors. Put another way, if you combined a beauty dish with a softbox, you'd have a parabolic umbrella.
This umbrella has a unique design structurally as well. The shaft is double walled for strength, but it tapers to 7 mm to fit Elinchrom and other European flash heads. Instead of bendable metal, the 16 inner ribs are made of durable fiberglass and the fixed black backing ensures that all of the light output is on your subject instead of through the umbrella. The white interior has a soft, neutral quality of light without specularity, while the black backing prevents light loss and flare due to back spill. You can vary the light-to-umbrella distance along the shaft for a harder or softer light.