I vote for the quark tactical AAx2. I have one and love it. I ordered a flush switch for $9 so it can tailstand.The "usual suspects" in the 2xAA scene. Let's say "extreme" means "more than 10 hours".
- Fenix E21: 150 l (2 h) / 48 l (11 h). I think the low is not that low. I have a TK20 with similar numbers and the difference is small. Great price for a quality light.
- Fenix LD25: You get 3 different 2-mode groups
* Outdoor mode: 180 l (1.45 h) / 45 l (13 h)
* Camping mode: 3 l (74 h) / 85 l (6 hs)
* Rescue mode: SOS (45 lumen), 85 l (6 h)
Keep in mind some people reported using lithium batteries and even fresh alkalines cause fenix lights lose the low modes until the voltage drops a bit.
- Quark AA2 tactical. You can choose your 2 modes from the list:
* Moonlight: 0.2 lumens for 30 days (1ma)
* Low: 4 lumens for 5 days (10ma)
* Medium: 22 lumens for 24 hours (50ma)
* High: 85 lumens for 5 hours (250ma)
* Max: 205 lumens for 1.3 hours (700ma)
* Strobe: 205 lumens for 2.5 hours
* Beacon: 0-205 lumens pulse for 18 hours
- Jetbeam BA20: 270 l (1.45 h) / Starts at about 40-50 lumen, then 20-30 lumen (35 h)
Of course, runtime may vary depending on the cells used.