I've discovered that three of my most recent posts appear on the Campaign Wiki Links to Wisdom. I am honored by whomever thought my ramblings were good enough to be included.
So when it comes to energy types, I usually think what d20 and later games call the five types of energy: acid, cold, fire, lightning, and sonic. My definition is broader, but of those five, my favorite type of energy is sonic. Why sonic? In part because it is used so sparingly. Very few magic items are created to resist sonic attacks or even produce them. For most folks, a sonic attack is a variation of a banshee's special attack, some kind of loud shout.
Like Sean said in the blog post, though, it's also the energy of vibration. That lends itself to earthquakes and tremors. Tweaking it a bit further, a creature's ability to use echolocation (like bats and dolphins) can also be transformed into a type of detection magic that also causes damage. It is the perfect attack against an invisible creature, especially for those creatures that have poor eyesight anyway (like bats and dolphins).
Having said that, I have a much broader list. It may be influenced from 4e, of all things. I still have acid, cold, fire, lightning and sonic. I also consider arcane magic as a separate type of energy as well as divine magic and psionic energy. So I consider Magic Resistance to be the same as Fire Resistance, it's just that Magic is a different form of energy.
In most games, arcane/divine/psionic energy is the same, block one and you block them all. With the right group, though, I make them all different. This makes spells like Resist Divine Energy and Block Psionic Energy necessary. It also make the game more dangerous. Bwa ha ha.
I also use darkness as a form of energy - it is the source of level drain and ability score drain. You know, those nasty shadows can take your Strength down to zero.
Lastly, there ectoplasm, specifically the matter that comes from the deep ethereal plane. It isn't energy as much as proto-matter. With the right magic or technology, ectoplasm can be shaped into any object or form of energy. I consider paladins and theurges of Ptah (remember Spelljammer?) to increase their mastery over time of converting ectoplasm into various things that appear in our world as spells. In a sense, they have only one spell, Summon Ectoplasm, but they learn endless variations of it to heal, produce flame, create food and water, and anything else imaginable.
Even with the extended list, sonic is still my favorite. Maybe in the next monster book, I'll find some sonic-based monsters.