Don't own a single one and I kinda doubt that I will until they pull off the Southard comp lock flipper, but, just based on handling them, the Mantra 1 by a long shot. Nothing against any of the following knives, but the Southard, Positron and Rubicon all feel like a knife that's gotten the Spyderco treatment, not a real Spyderco knife, if that makes any sense. The Domino and Dice ended up, to me, kind of feeling like "Me too!" knives. Very good, but knives that felt like they were put out to cash in on the flipper trend rather than designed as flippers from the ground up, again, just my opinion.
For me, the Mantra 1 is the first real Spyderco flipper. It's undeniably a Spyderco with nothing extraneous, no bells or whistles that have become so in demand, just a knife that feels great in the hand, flips open fluidly and looks like it will be one hell of a slicer. There are some definite exceptions *cough* Chinook *cough* but in-house, function before form knives are kind of 'home' for me where Spydies are concerned and the Mantra 1 does that better than any of their other flippers so far.