This is a silly question unless you play 3e or 4e. In the versions of D&D I enjoy, there is no firm taxonomy of ‘energy types’ (like “radiant” and, um that’s only one I remember…). We throw around loose terms like “fire,” “cold,” “electricity” etc. and somehow manage without defining them, although I can actually see why some game designers thought there was a problem. Look at the description for a ring of fire resistance in the 1e DMG and it talks about three different intensities of fire (normal fires, very hot fires, and exceptionally hot blasts) so maybe there was a DM somewhere who needed more guidance about that sort of thing?
There was such a thing as negative and positive energy in 1e, which came from the negative and positive material planes. These planes were never described in any detail as far as I know but the more powerful types of undead, particularly those that drain levels, have a connection to the negative plane and use negative energy for the drain. I have always thought level draining made undead truly scary in a game already filled with weird monsters and while I sympathize with people who say energy draining is a bitch, I think it still has a place in the game. (I would just make it a little easier to recover from rather than eliminate it.)
So I guess my favorite energy type is negative energy.