Rubber boots are for the days when the weather's not up for negotiation. Dog walks in sideways rain, festival fields, school runs through puddles that have clearly been building a case all week, standing at a bus stop while the sky does its worst. They're not trying to be clever, they're trying to keep your feet dry, and a good pair does that without any fuss at all.
Black is the one that goes with everything and hides the inevitable mud splash up the side, but I've had more use out of a dark green pair than I expected, since it reads as outdoorsy rather than wet weather gear and doesn't look out of place with jeans on a normal Tuesday. Navy works the same way. Bright colours are lovely in a shop and then you realise they show every scuff within a fortnight, so go in with your eyes open on that. For fit, you want room to get a thick sock in without your foot feeling strangled at the ankle, and you want to check the calf isn't so snug it leaves a mark after an hour, because rubber doesn't give the way leather does, it just sits there being firm about it. I made the mistake once of buying a pair that fitted perfectly in the shop with thin socks on, then couldn't get a welly sock in come December. Try them with what you'll actually wear underneath, not what you happened to have on that day.
These earn their keep from autumn through to the soggiest bits of spring, and they're brilliant for anything outdoorsy, a wet garden, a muddy gate, a dog that needs walking regardless of what the sky's doing. They're less at home anywhere you'll be standing on hard flooring for hours, since there's no give in the sole and your feet know about it by lunchtime. If you've got none of these, start with a plain mid calf pair in black or green, nothing fancy, no lining you don't need unless you genuinely get proper winters, just a boot that does puddles and mud and doesn't ask anything else of you. The honest limit is comfort over distance. They'll keep you dry standing still or walking steadily, but they're not built for a long day on your feet, and your ankles will tell you so if you try.