With that background out of the way, let's look at the spicy ones you may already know and the ones you probably havent thought about until now, and don't get caught up in the ranking if your favorite isn't #1, what's important is that you win
Also, low mana value changelings do exist
In the Theros versions, the means two different things: colorless in the mana ability, and generic in the cost of the activated ability
Gond Gate has the ability to make your Gate lands more powerful by allowing them to enter untapped, and when it is tapped, it also grants you a single mana that one of your controlled Gate lands could produce
Lilianas Contract lets us cash in some of the lifegain from Solkanar to draw four fresh cards, and then has an alternate win condition that we can work towards over the course of the game