Gosh, finally an interesting question on this list! I love energy types, they’re my favorite part of the game! I can’t imagine how we ever managed without them. I mean, when a fire giant that is immune to fire is attacked with a torch, does he get damage? How do you know if the torch is doing fire damage? Leaving it to the DM seems unfair and untrustworthy. Fortunately, energy types were introduced in 3rd Edition, and 4E made them even better! And I'm liking what I've seen so far in Next.
Anyway, my favorite energy: I ‘m a big fan of Necrotic energy and Radiant energy. Necrotic comes from the negative energy plane, and Radiant comes from the positive energy plane, but the designers were smart enough not to call these energy types “positive energy” and “negative energy”. That would have been stupid. “Necrotic” sounds much awesomerer!
However, the very best energy and my true favorite is Fire energy, because it can set things on fire and burn things. Now that’s what I call awesome!