You're asking the wrong question (although you're asking the question everyone asks).
What features do you want? Portability? No compromise fidelity? Are you more interested in the intricacies of the build than the final product?
I've built 5 amps now (M3, Millet, PPA, Pimeta and Pint) aside from my Cmoy training amps, and each one has it's place, except I could live with only 4
If you want portability, the argument would probably get down to Pint vs Pimeta and there are valid arguments for either.
If you want the best thing you can power with batteries and pick up with one hand, probably a PPA with battery board.
If you want the best desk top fidelity, the argument would get down to Gilmore Dynahi vs M3, although I cannot imagine that the Gilmore brings anything to the table (soundwize) worth the extra effort and difficulty vs M3. I have not built a Gilmore but I have read the build threads. My view of the Gilmore is that it is a "novelty" in the sense that it is so extreme it is very difficult to build. The build process is probably the attraction there; simply sourcing all the parts and getting it to all work marks you as a "Builder Supremous".
I went for the M3 for my no-compromise desktop amp. It has an insane amount of power for headphone use and drives small (edit:speakers). It is flat from DC to the speed of light, distortion of 0.001%, and it sounds like it specs. You can get all the parts from the usual suppliers at a very reasonable cost.
If you have any interest in tubes at all, you almost have to build a Millet.
Edit: Tomb, you are right about the Mosfets. The M3 is a joy to bias out to the extreme. I did a lot of testing of Millet discrete buffers and the combination of the postive coefficient BJTs and the extreme compact design (with two pairs of output trannies) makes an inherently unstable circuit. Anyone that disagrees with me has probably not bothered to monitor their bias while it is cased up and unventilated.