My first set of cloths came as a pack of six from a discount bin, thin enough to see the table through, and they taught me that a cloth with no weight to it just slides around under plates and bunches up the second someone leans on the table. That's really the whole lesson. Weight and weave matter more than pattern ever will.
A busy printed cloth fights with everything else doing work on the table, the plates, the food, any centerpiece with color in it. If your dinnerware has a pattern or a rim of any kind, a plain or lightly textured cloth is the one that won't compete with it. Linen or cotton in a solid color or a subtle check is the safest ground. Save the bold prints for a table that's otherwise completely plain, or skip them and let a runner carry the color instead.
Getting the size right is where most people guess and get it wrong. The cloth should drop somewhere between eight and fifteen inches past the edge of the table on all sides, measured with the table actually set up the way you'll use it, leaves in, extra boards in place. A cloth bought for a four seater disappears into nothing once you open the table out for guests, and one sized for a huge table pools on the floor and gets caught under chair legs all night. Measure the table at its biggest, not its everyday size, if you ever stretch it for company. And if you're between sizes, go up. A little extra drape looks intentional, a little short looks like a mistake.
In an ordinary room, a good cloth sits well with plain dinnerware, mismatched glasses, and whatever candles or jars you've already got rather than anything bought specially for the occasion. It doesn't need napkins that match exactly, a close enough color is fine and looks less stiff than a full matching set. Cotton and linen both soften with washing and actually get better looking over time, which is more than you can say for most things that live in a drawer.
One more thing worth saying: a cloth that needs ironing every single time will quietly stop getting used, and then it's just folded fabric taking up space in a cupboard. Before buying, I'd ask myself whether I'm actually going to wash and press this one regularly, or whether I want it for the one dinner a year it gets used.