Whatever your rim and hub measurements dictate.

The spocalc.xls spreadsheet and DT Swiss website calculators are convenient

Whatever butted spoke you want with 2.0mm elbows. 2.0/1.8mm aka 15/14 gauge are the best all-around choice for durability and ease building. DT Swiss have reasonable length elbows for good support and a nice smooth transition between thick and thin sections. You won't make a real performance difference with thinner/more expensive round spokes but can have bragging rights.

1.8mm elbows are more than strong enough although modern hubs have 2.5mm holes to clear the 2.3mm threads on 2.0mm spokes which can leave part of the bent elbow both unsupported and in a position which can't be stress relieved thus causing fatigue failures until you respoke with heavier gauge spokes.

2.0mm straight gauge (one thickness) (DT Champion) - least expensive, easiest to build with, heaviest, less durable than a double butted spoke because the stress is concentrated in the elbows and threads. With double butted spokes generally lasting until you can't buy rims with the same ERD, 2.0/1.8mm still easy to build with, and the price difference about $.20 a spoke I wouldn't make this choice.

2.0/1.8mm double butted (the ends are 2.0mm, the middles 1.8mm) (DT Competition) - a little more expensive, a little more windup, a little lighter, and more durable since the stress is concentrated in the straight section.

2.0/1.7mm/1.8mm triple butted (elbows are 2.0mm, middles 1.7mm, and threads 1.8mm) (DT Super Comp) - much more expensive and lighter still. They might make sense for the drive side of a rear wheel with 24 or fewer spokes for a heavy rider who wants to pay for light but wouldn't like the stiffness from a thinner spoke but otherwise seem to be an answer in search of a problem.

2.0/1.5mm double butted (DT Revolution) - costs the same as 2.0/1.8 butted, lighter still, about half the torsional rigidity as a 2.0/1.8mm spoke so windup is an issue when building, strong enough. A good idea for non-drive side spokes on rear wheels with soft/light rims (Stans Alpha 340) that won't tolerate high drive-side tension especially on hubs with a steep drive side bracing angle (like Powertap) but you'd be smarter to avoid those configurations in the first place. May have unacceptable lateral stiffness when used on the drive side in low (24 and less) spoke count rear wheels for heavier riders. Garmin wheel magnets don't stay parked as well as on a 2.0/1.8mm spoke.

2.3/0.9mm aero (DT Aerolite) - 4-5X as expensive as a round spoke, looks cool, may save 5-10W at speed, fits through round hub holes since the flat portion is the same width as the spoke threads. They start life as 2.0/1.5mm spokes and therefore have the same issues with lateral stiffness.

These guys have good prices, a spoke cutter, and no additional charge for odd lengths

http://store.icyclesusa.com/competition-c2613.aspx