Having been through a number over the years, it is Dewalt 18 volt lithium every time.
See this page for a good explanation of the different types:
http://www.its.co.uk/blog/buying-guides/dewalt-drill-buy/
Dewalt basically do three levels - the cheapest is often available from the likes of Screwfix on offer around £90 (depends on size and number of batteries). These have a lot of plastic components and are perfectly fine for occasional DIY. I've bought my daughter one of these. The next level up (700 series) is a good professional level drill for everyday use (I have two). The 900 series are the mutts nuts of the range, but probably only sensible if you are using it on site every day.
It's always better to get two batteries than one, even if they are much smaller capacity. My chargers tend to charge faster than I can use the battery up, so one on the drill and one charging gives you continuous use.
I don't know about the drills, but I have a Dewalt multitool with their new brushless motor, and that is well worth the money - it seems very solid and responsive.
Whatever you go for, don't buy a NiCd battery drill as the batteries will be dead within three years, and unless you are in a hurry, keep your eyes open for deals on sites like toolstop.co.uk and ffx.co.uk.