A good stacking ring sits in your palm with a bit of weight to it, not a coin exactly, but not air either. You can feel the join if you run a thumbnail round the inside, or you can't, and that's the first clue about what you're holding.
The difference between a ring that lasts and one that turns your finger green within a fortnight comes down to a few boring but important things. Solid metal, even a thin band of it, behaves completely differently to a hollow one or a thick plate over brass. Solid gold or sterling silver has a give to it, you can feel it settle against your finger rather than sit stiffly on top of it. Plated pieces can look identical on day one, sometimes better, because the finish is new and shiny in a way solid metal with its own slight texture isn't. The plating is the whole story though. If it's thin, it wears through at the knuckle first, usually right where the ring moves most, and you'll see a dull patch or a flash of a different colour underneath within a few months of normal wear. Check the inside too. A cheap band often has a rough seam where it was formed, you can feel it catch on the way past your knuckle, and that little snag only gets worse. A properly finished one is smooth right round, no ridge, no sharp edge where the ends meet.
I bought a set of five gold plated rings years ago, convinced I'd found a bargain, and wore them washing up without a second thought. Two of them had gone a sort of bruised grey at the join within a month. That's the thing about stacking rings, they're small enough that you stop noticing them, which means they go through everything your hands go through, hot water, hand cream, gripping bags, all of it. A solid band shrugs that off. It might pick up a soft scuffed sheen over time, which honestly looks better than new, like it's been lived in rather than just bought. Keep them out of chlorine, take them off for anything with bleach in it, and give them an occasional polish with a cloth rather than anything abrasive, and that's about as much looking after as they need.
If you've got none and you're starting from nothing, go plain first. A thin, slightly rounded band in a metal you already wear elsewhere, no stones, no texture, something that can sit alone or stack with three others without an opinion of its own.
Over the years the edges soften slightly and the surface loses that brand new flatness, and that's when it stops looking like something you bought and starts looking like something you own.