Quark Mini CR2 (XP-G) on my keychain as a backup for anything I carry (or forget to carry).... I suppose it's borderline not-budget (although it is under $40 shipped), but probably the best money I ever spent on a complete flashlight (best money I ever spent on a flashlight ITEM so far was my Manafont 3-mode smooth reflector XM-L drop-in). Saw a few previous mention of 4sevens Quark products in this list, including the CR2.
When it's cool enough to wear a jacket, since XM-L drop-ins came out I've carried some variant of Surefire 6P clone (either Solarforce L2 or Ultrafire WF-504B... very similar).
If no jacket or I don't feel up to carrying an 18650 light, it's my Trustfire TR-Z1 (set to low, in case it goes on in my pocket--which it did once), which is something of a takeoff on the original Zebralight H30.
Again, if no larger flashlights are up to the job, the Quark Mini CR2 and its nearly 200 lumens does a remarkable job, and since it's a real 3-mode, medium and low are available if longer runtimes are needed. I carry a CR2 primary in my car and keep a LiFeP04 rechargeable in the light (going to purchase a li-ion, they're just a bit hard to find; Lighthound has them).
Dishonorable mention: MXDL "3W" 16340 from DX. Has the "overeasy egg" LED. Yes it DOES pull 3 watts--but the question is, what does it DO with those 3 watts. And the answer is: not a lot (at least not from the perspective of a budget flashaholic). Inefficient, weak, bad/blueish tint. Terrible by today's standards, even with low expectations for anything 3 watts, even if it was only $7 or so.... $7 wasted. Bought it originally for to screw a P60-style drop-in pill into the head, but the threading stops partway down on all the drop-ins I own. Apparently DX's MC-E drop-in used to have threading which went all the way down the pill, enabling it to screw into the head, but I think I read they're not like that anymore, and price on the mc-e drop-ins are ridiculous today as the prices haven't really gone down, and the drop-ins reportedly didn't drive the MC-E very well in the first place, even with a LiMN/LMR 16340 (or even 18650, according to my friend who has one--pulls only like 1.4 amps, can't remember exactly). I'm not even sure if it's good enough to gift/pass along without embarrassment to both parties. And it's not as small as I expected.
I've concluded that you must not have antipathy towards your keychain light, because you have to see it/feel it multiple times a day. So the Quark Mini CR2 fits the bill. I wish they made a reflectorless/even smaller "mule" version.