adizero PR-- my favorite flat. Fantastic fit, super responsive. Perfect for a 5K or 10K. I'd be interested in trying them for a 15K or half-marathon. I suspect they would hold up OK, but I haven't done it yet.
MayFly-- incredibly well-cushioned for how light they are, and a lot of fun to run in. I'll do some speedwork in them, but I like flats with a less-cushioned, more responsive feel for actual races. Also, the upper separates from the sole pretty easily. I tore a pair of mine weaving through pedestrian traffic in New York.
Zoom Streak XC-- my flat of choice for races longer than 10K. Little bit more cushioned than the PR but not bouncy like the MayFly. Very flexible shoe, which I like. I also like unstructured upper & the fairly wide forefoot-- a lot of flats feel too narrow.
Katana Racer-- fits just like the Streak (which I like), but the sole feels much stiffer. Also, the outsole is sharp & tends to leave scratches along the inside of my ankle. Not a big fan.
t4-- I haven't worn the t4, just the t5, but the sole looks the same. I didn't like the fit. There's a weird stiff spot along the arch. Looking at the outsole, it's right where that lug is that has the Brooks name on it.
No experience with the Piranha or the Streak Vapor.