I own a Nitecore D10, Fenix L1D/L2D Q5 and now a Quark AA2 Turbo and have some thoughts.
As always, it depends on what you like and your needs. To me, my EDC remains the D10 because I LOVE the piston drive! It is also smaller than the Quark AA so that has to gain a point or two.
I use a L1D RB100 and L2D Q5 as bicycle lights. Taking note of selfbuilt's reviews of both lights, the Fenix L2D or L20 has a longer runtime than the Quarks at about the same brightness. The Quark does look better as Fenix tried modern day fugly when doing the L2D series. If runtime is important to you, the L20 will run longer on max output but that target is moving. I'm interested in what Fenix does with the XP-G to make it different from the Quark (I'll bet you the Quark and Fenix have the same parents!)
I've had my Quark AA2 Turbo for 22 hours now and have some thoughts. A very tough, heavy duty and bright 2AA light with better throw than the L2D due to the large reflector. The smooth reflector can throw but it does add a little "donut" ish part to the beam. Not a big deal for me since it is used for cycling outdoors, indoors it might bother you more. The tactical UI with head tight/loose for the two modes you pre-program is nice. I run max/medium on mine and it works well. The thing is larger and does have some weight to it. The manual says 3.6 ounces (102.2 grams) with the L2D Q5 being 2.1 ounces (60.7 grams)
Fenix better come out with their XP-Gs quickly to counter Quark, that is the battle ground. Nitecore has the piston drive and I will get an XP-G version to replace my ringier Q5 XR-E light. When you look inside the head of the Nitecore and then look at the Fenix/Quark, I just like the stronger/thicker brass contacts of the Nitecore better.
Quark might be ahead of Fenix right now but Nitecore is a different animal. The D10 and EZAA are in their own class but David will be introducing more lights for the next month.
One thing that is really cool no matter what of the 3 brands you choose, you can get them in one place with free shipping and a CPF discount