Reinforcing the Veritas suggestion. It's a bit of a pain to build, but pretty difficult to make leak (and flavorful as hell; totally love mine). The Marquis works the same way, I believe.
The Achilles is an awesome rda, but absolutely only for mouth-lung draws. Just zero lung hit capability. Pretty perfect for what it is, though (and more or less leak-proof).
The Holmes is similar to the Achilles, but lets you dual coil and features an adjustable and potentially really airy draw. No leaking, either.
The Mini and 22mm Yaeliq rdas work similarly to both the Achilles and Holmes, too (so leak-resistant). I own a Mini for Egos and iSticks and such, and I swear by that thing. Tighter draw, though.
A running trend you notice here is that most leak resistant rdas bring air in from under the coil. As such, they typically give great flavor, but it can be harder to find one with a really wide open draw.
Edit: forgot that Chels of Mini-Ecig just put out two drippers (a 22mm and a 16 or so) which are designed comparably to most of those and, as such, oughta be hard to make leak. I actually have a Microdrip in the mail (fiiiiiinally they ship), so I'll letcha know what's what when it gets here. Gonna be hard to unseat that Mini-Yaeliq, though...