The best batteries available are the ones with the highest mAh/hr. They will last the longest
IF used immediately after a charge. If they sit in the detector for days and weeks not being used, they start to drain by themselves, reducing the usable time. Also, a brand new set will hold a charge a lot longer then a set which has been charged numerous times. Now, there are rechargeable batteries available which will hold a charge in them for 6 or more months. They are usually a combination of alkaline/NiMH rechargeable batteries. They don't usually have the high mAh/h as the plain NiMH do, and don't fair as well in cold temperatures. For really cold temperatures. the lithium batteries (non-rechargeable) batteries work well in extremely cold weather.
What to buy? Stick with a known manufacturer, as there are many batteries available on line with a high mAh/hr rating, at bargain basement prices. Often times, these batteries do not perform well at all. DO get a higher mAh/hr if possible. If you hunt 1-2 x a week, the alkaline/NiMH batteries with the long shelf life will serve you well. If you hunt more often, a high mAh/hr is the way to go. Expect to get about 2 years worth of decent hunting battery life with a set of rechargeable batteries.
Hope this helps and good luck!