Hi, Sybert! Long time no see! Well, the ideal light is going to be subjective. Do you mean an EDC light? A light for camping? Different lights will be better suited to different purposes. But there are some generalities, at least for me.

First off, it has to be LED. That may go without saying in this day and age, circa 2016, but I have no use for incans anymore.

Next, my strong preference is for CR123a batteries. I'll concede that AAA/AA are useful for some applications; I often carry a Streamlight MicroStream AAA with a lithium cell in it as a backup to my regular EDC light. And AAA is great for keychain lights to keep the size down. But for my go-to light I absolutely require the use of CR123a cells!

I strongly prefer lights with simple UI's and just a couple of modes. For 95% of what I use a light for just a HIGH and LOW will suffice. A low mode is sufficient for getting my keys in the door, looking around under the seat, checking the mailbox, etc. Depending on the output of the light LOW is often enough for most tasks. But I like to have a HIGH mode for camping, navigating parking lots, urban use, "tactical" use, etc.

The UI (User Interface) if critical IMO. I want the button to do just one thing, turn the light ON and OFF. The HIGH and LOW modes should be accessed by twisting or rotating either the bezel or head. Elzetta calls this "uncoupled binary controls" and it's my ideal UI. My second choice- and let me add, not a close second- is one button that toggles between HIGH and LOW. This is suboptimal IMO but I can live with it as long as the light defaults to/comes on in HIGH.

Two CR123A cells is my preferred for factor for most uses. A singe CR123A can be nice for EDC.

My favorite lights right now are my Elzetta Alpha and Elzetta Bravo's. Usually I'm carrying the Bravo lately. I have two of them, one with an AVS head and one with a Malkoff M61N dropin.