Avenger of Zendikar is under $2 and found in tons of precons
It has value as a collectible, its gameplay is accessible but also allows for interesting strategies, and it has some of the nicest-looking cards of any TCG
Seven power for 5 mana is already outstanding on its own, but giving haste to other dinosaurs sets Regisaur Alpha apart
Being able to ping any target based on the power of ANY creature that enters the battlefield under your control is an incredible boon, especially if your deck is chock-full of high-powered threats that can snowball incredibly quickly
Be free to break those rules and differentiate as you see fit, as many times special builds/decks require quite different approach and it heavily depends on your preferred playstyle, the chosen commander and the mechanics in use, among other things as well
Landing at number five on this list is an impressive feat for any Supporter let alone one with more niche appeal