OK - ultimate light is one that is versatile, powerful, and most useful for the most general set of circumstances. And maybe also upgradeable!
So I nominate the SF 6P formfactor (I say formfactor because you can get it, as I do, in a cheaper, clone version).
You can get all sorts of tailcaps for it (infinitely programmable, tactical, strobe, simple on-off! whatever you want), different battery tubes (so you can use 2xCR123A, 2xRC123A, 1x18650), tactical or normal bezels, and, the most important, all sorts of different drop-ins.
You can get easily now the Q5 CREE LED drop-ins, you can get them in a throw or flood configuration, or have the best of both worlds (I have the Malkoff's, with the big throw, plus an SF diffuser which can transform the beam into a nice even flood). And you can choose lumen outputs all the way out to 235+.
As LED's develop, you can keep getting new drop-ins, enjoying the best of the best at a minimal outlay.
On top of which, the 6P clones are cheap, and you feel COMPLETELY FREE to use the light under all circumstances--normal and hazardous, domestic and while traveling (if TSA wants to confiscate it, no prob--OK, maybe a small prob.!). You can even do a bit of protective modding to make them fairly water resistant.
Note that those that offer the expensive 1xCR123A formfactor EDC lights point to one or another advantage (programmable for example), which is also available if you want or need it for the 6P formfactor--just by choosing the right drop-in or tailcap. (infinitely variable tailcaps and drop-ins do exist). The claimed advantage of multiple types of beam shapes (for some lights) can just be had by using the throw drop-ins plus the diffuser. If you want a specific color, you can get a filter for that too (even IR).
Finally, there is the issue of great runtime and POWER--this formfactor can put out the big light for reasonable runtimes, so it is the most appropriate for general flashlight use. It fits great into your hand, without the need to carry the 3xD maglite you'd have used in the past to do the job. The small EDCs are kind of more annoying to handle in your hand, less natural somehow.
Of course, sometimes you just want that very small versatile light (and arguments can be made in favor of the 1xCR123A formfactor, but also in favor of 1xAA or even 1xAAA--if smallness is an advantage, isn't smaller better?). But then that is a specialty use, not the ULTIMATE!