Fenix and 4Sevens both make quality lights. One of the great things about the 4Sevens lights is that after you get a couple of them, you can Lego them because their heads, bodies, and tailcaps all interchange. As long as you're careful to make sure you're using the proper voltage with a given head, you're good to go. You can whip up all kinds of fun combinations with them for just about anything you can think of.
I'm personally fond of Zebralights myself. I've been very pleased with them all the way around. They have great tint, durability, beam profile, and tint. I feel their user interface is the best of any light I've used. They don't give you the same kind of flexibility as the Quarks though; each is a dedicated unit and there's no way to mix and match their parts in any meaningful way.
I've also had good luck with Jetbeam lights too, though they seem to not be making the same kinds of neutral tints that I like. One good thing you might want to check into is the Xeno E03 or E15. They nice little lights, and a good value. I think they'd be a good choice since it seems like you're still just getting started with LED lights.
The E03 is a dedicated 1xAA light and is available with cool, neutral, or warm tint emitters. You can use a li-ion 14500 in them to make them into a real pocket rocket, but they give decent performance on 1xAA as well, and longer run times. They aren't regulated though, so as the battery depletes they get progressively dimmer. They're also not anodized, but painted instead. But, they're about $35 and a good value for the price.
The E15 is a fancier version of the E03. As far as I know it uses the same set of emitter options and driver, so it will perform just like an E03. The difference is it comes with an extender for the body, so you can run it as either a 1xAA or 2xAA light. If you don't use the extender, you can run them on 1x14500 just like the E03. Personally I think the E03 is a better value. Unless you really need more run-time, and for biking you probably do, then the E15 is the better way to go.
If the reducing brightness is an issue for you, you'd be better off getting a different light. A more expensive light will have regulated output, meaning it will stay at a constant brightness right up until the battery is just about dead. The disadvantage is that some regulated lights will just up and shut off, which could be a problem in the middle of a downhill...
Like anything else though, there are ways to deal with that.
The best thing to do is write down what your intended use is going to be, and what kinds of demands that will make on your light. Once you've done that and shared that here, other members can give you better and more specific advice and suggestions in terms of what to think about to meet your needs.
