Horn of Ramos

The cleverness here is in the second ability, not the first. Tapping for a single green mana is filler: a worse mox, a colored Sol Ring that ramps nothing. The sacrifice line is what gives the artifact a reason to exist, because it lets you cash the permanent in for its mana on a turn when it has already done its work, or feed it to a payoff that wants an artifact in the bin. This is the design pattern Mercadian Masques built its whole cycle of Ramos-named artifacts around: each is a mediocre repeatable mana source that doubles as a one-shot when you need the artifact to disappear. The Horn's value was never the green it produced; it was the flexibility of being a permanent that could become an instant-speed sacrifice trigger and a burst of mana in the same motion. That makes it a fixture in builds that care about artifacts entering and leaving, where the mana is incidental and the sacrifice is the point. As pure ramp it is among the weakest things you can do with three mana; as a cheap, expendable artifact that quietly fixes a single pip of green on the way out, it has found homes that a stronger mana rock never would.