With that said, for me this list begins with Aethersquall AncientAethersquall Ancient, which while expensive, is the threat that I have seen win energy decks the game time and time again
Marcy: Probably one of the more interesting mana dorks, in that you get Energy, not mana, but the trade off is also that you get to buff all your creatures
Besides creatures, Hardened Scales , Branching Evolution , Tribute to the World Tree , Court of Garenbrig , and Ozolith, the Shattered Spire all give you more +1/+1 counters (sometimes a lot more), while The Ozolith ensures all those counters never go to waste even if the creature they're enhancing dies
This cheap artifact boosts each instance of energy creation by one, which ends up being a substantial amount in the long run
One thing to watch out for, especially with older cards, is how creature types have changed over time