As others have noted, a helmet light is useful both day and night; if I had to use just one headlight it would be a good helmet light. You can see ahead into turns, more likely to get the attention of cell-phone stupefied motorists, useful while doing repairs, etc.

To put out enough lumens for a daylight headlight/helmet light you probably want to take the jump to rechargeable LiIon cells, either (or both) 18650's and the AA sized (but almost 3x the voltage, so do not use in typical AA light) 14500's plus a good, safe charger.

The general consensus on the Electronics subforum is that LiIon flashlights are the best lumens/dollar. The best charger (short of hobby chargers) is Pila's, but not inexpensive (it will charge both 18650's and 14500's). There are lower cost ones (see the Electronics subforum). Helmet mount the light with the Zefal DooDad, TwoFish BikeBlock or TwoFish LockBlock, depending upon your helmet's vent configuration.

Current favorites of the day & night cyclists in our family are the UniqueFire G10 CREE XP-G R5 6-Mode 350-Lumen White LED Flashlight (no, its not really 350 lumens but it is bright and has both a fast ~10 Hz and slow ~1.6 Hz flash rate) on the helmet and the MG P-Rocket Cree XP-G R5 450+ Lumens Compact LED Flashlight "OP Reflector" on the handlebars (no flash mode, though). Perhaps even better (latest LED generation) would be the one of ShiningBeam's lights using the Cree XM-L T5 LED.

If you are experiencing more close calls now, as previous poster notes, the sun is now low on the horizon for more of the day in northern latitudes, so glare is much more of a hazard than in summer. Remember the direction you cast a long shadow is the direction approaching motor vehicles may not see you due to glare.