Jeans have had a proper shake up over the last few years, and mostly for the better. For a long time it felt like every pair was engineered to hug, stretch and then sag by lunchtime, whatever the label said. Now there's real width in the leg again, denim with some substance to it, and a lot less pressure to go a size down and suffer. There's still some thin, flimsy stretch denim about that looks fine on the hanger and nowhere else, but the standard has gone up.
A decent pair earns its place by how little you have to think about it. Blue jeans go under a plain jumper and trainers for the school run or the supermarket, the same pair takes a blazer and boots to work without any fuss, and black jeans do the job a smart trouser would do on a day when you can't face ironing anything. I keep a white t-shirt and a soft shirt in rotation for exactly this reason, because the jeans change the whole mood without me having to plan an outfit around them. They're also the thing that rescues a nice top you don't know how to wear, because they don't compete with it.
If you're starting from nothing, go for a straight leg or a slightly relaxed fit in a mid blue wash before anything more fashion led. It sits well with most shoes, doesn't date in the way a very skinny or a very exaggerated wide leg can, and forgives a bit of bloating or a big lunch without cutting in. Skinny and wide leg are both worth having eventually, but they're the second and third pair, not the first.
Good denim ages well if you let it. The first few washes soften it and settle the fit, often for the better, so don't panic if a new pair feels stiff or sits slightly big in the waist for a week or two. Wash them inside out, on a cool setting, and less often than you think you need to, because every wash pulls colour and stretch out of them. Turned the wrong way out, they'll keep their shape and their shade for a long time, and the knees and seat will tell you honestly when a pair is finally done.
They suit most of the year, from a cold morning with boots and a coat to a mild evening with sandals, and they work for nearly every ordinary occasion short of a proper formal one. What they won't do is replace a going out trouser or carry real heat well on a hot day, denim is just not built for either.