I have every original size Quark combination there is (aside from AAx2, which I have no desire to have.) Includes XP-E, XP-G, Cool and neutral emitters, tactical tails and heads, regular tails and heads, 123, AA, 123x2, 18650, etc. The lego possibilities can keep you busy playing for days. No Ti, though, as they just never seemed worth extra money for my interest and apparently they tend to gall. Some folks do like them, though.
That said, if I could only have ONE as a general purpose light, including pocket-light/EDC use, my choice would be a AA, neutral tint, XP-G R4, tactical.
Not too big, not too small, virtually universal powering source size running on 14500 li-ion, AA-size NiMH, AA-size Eneloop, AA-size lithium primary, and the lowly-but-ubiquitous AA alkaline. Simple and programmable two modes with easy and comfortable to use momentary forward clicky.
Add a standard tailcap if you want tailstanding. Add other bodies and heads as you become sucked into the Quark world of flashlight lego. :devil:
That said, I love the 123x2 neutral XP-G R4 tactical running on a 17670. I only use li-ion to power my Quarks, unless I'm far from home and have to temporarily use other options. The 123 is another favorite... well, you get the point by now, right?