Mexican · Espolon
A paloma has less to hide behind than a margarita, because grapefruit soda is sweeter and softer than lime juice. You want a tequila with some pepper in it, and Espolon has exactly that.

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The one thing that matters
One hundred per cent agave at a price that makes a jug sensible. This is the everyday paloma bottle.
A blanco with real agave character. Espolon works because it has pepper and citrus that carry through the grapefruit soda. A very smooth premium blanco can get lost.
Either, and unlike a margarita the reposado genuinely works here. The oak sits well with grapefruit. Try both.