My top five powerful cards list, in no real order, is probably Sol Ring, Black Lotus, Recall, Time Walk and Academy
You could build around the legendary text or completely ignore it, though you'll probably want to lean on equipment for keywords like lifelink and deathtouch
Monkey.D.Luffy ST08-001 (One Piece Starter Deck 8: Monkey.D.Luffy | One Piece Cards) prices are based on the historic sales
Mister Fantastic The passive card draw on Mister Fantastic is already a great start, and the ability to double-copy a triggered ability has all sorts of uses, from infinite combos to strong value chains
Unearth only as a sorcery.) Creature Artifact - Horror Phyrexian Sword of Truth and Justice Equipped creature gets +2/+2 and has protection from white and from blue
Grading MtG cards can be a costly exercise with minimal return, due to the fact that graded cards are returned in tamper-evident cases and thus need to be kept out of play in order to retain their grading, as well as the risk of rare cards being reprinted in newer sets and dramatically dropping the price