I've tried a bunch of different seltzers, but the ones I consistently come back to are the "classic" La Croix flavors.
Lime, Lemon, and Grapefruit baby. The more exotic ones are nice every once in a while, and other brands are cool, but I always circle back to where it started.

Topo Chico Lime/Grapefruit is flame, but hard to find where I live. Expensive compared to La Croix too.