Smart-alec, but true answer: Short ones. Remember the 90 degree shoulder rule to keep from being too stretched out.

Some folks find that the small Syntace C2s are short enough. Most move to something like the Profile Jammer GTs if they need more adjustability. A very good option, and one that I use is the Deda clip ons. They're very adjustable for length, and the armrests are standalone which provides maximum adjustability. You can also buy them in standard sized clamp or OS to fit the 31.8 oversized drop bars if you have those.