There is 31 one mana cards in the M20 set with some reprints and quite a few original cards. After reviewing them all, there are quite a few useful cards, and lots of value to be had for one mana. There are four that stand out above the rest.
Elvish Reclaimer - It is not easy to get
Elvish Reclaimer to become a 3/4 creature in the early game, it becomes more likely around turn 4 or later unless you very specifically set up your deck.
Stitcher's Supplier can help, plus anything with Surveil. By time you do get the boost, there will be other 3/4 creatures or better on the board. The better use of this card, is to take advantage of the ability to sacrifice lands to bring in special lands, if you notice it says search library for a land card. You can build a strategy around that. It just so happens that if you pull that off a few times,
Elvish Reclaimer gets a nice boost. This is a different approach to mana fixing. I suspect this will see play in competitive play before M20 cycles out of Standard.
Knight of the Ebon Legion - The return of the pump knights!!! With two abilities, the first feeding the second, without interaction this Vampire Knight can win the game single handedly.
+3/+3 AND Deathtouch for 2B. Without a tap as a cost you can keep paying that to grow larger. Second ability helps
Knight of the Ebon Legion grow by any player losing 4 or more points of life in a turn. ANY PLAYER!! Don't let this card stick to the battlefield if it isn't on your side.
Spectral Sailor - 1/1
Flash and Flying; in addition to being a Spirit and a Pirate.
Spectral Sailor fits nicely into two popular tribes and can be a great card in both. Why? Because 3U: Draw a card, that's why! It is really hard to say, but between
Spectral Sailor and
Knight of the Ebon Legion there is a tie for the best one mana card in the set.
Vampire of the Dire Moon - Of the 4 cards I have chosen to talk about in this article, 2 of them are Vampire creatures. Vamp DerMoon here, gives you Deathtouch and
Lifelink for B. That is an incredible deal. Nothing like this has been printed before, except
Basilisk Collar way back when…. And that was not a creature, and still had to be equipped. So easy to drop, enchant, equip, and just beat with from Turn 1 forward. Great card.