Note that this white enchantment states power 2 or less, not mana value, and there are plenty of powerful creatures that fit that criteria
Galerider Sliver takes the first slot on this list given its ability to give all your Slivers a huge advantage with their aerial superiority
My proof of concept that kept me watching for ways to abuse it was the combination of Emmara, Soul of the Accord, Song of Freyalise, Shanna, Sisays Legacy and March of the Multitudes, which alone is lethal on Turn five (assuming no missed land drops)
Heres an early proposed list: Creatures (25) 4 Psychatog 2 Putrid Imp 4 Mesmeric Fiend 4 Ichorid 2 Dark Confidant 2 Dimir Infiltrator 3 Golgari Grave-Troll 1 Golgari Thug 3 Stinkweed Imp Lands (20) Spells (15) Remember, this was before people thought about Dredge as being a mechanic that lets you skip your draw steps in order to rebuy cards youre not going to play
When this card enters, you can prevent all damage to one of your creatures and turn that damage into +1/+1 counters for that creature