Most people buy straight leg trousers in their usual size and wonder why they sit oddly at the hip or pool at the ankle like they forgot to finish getting dressed. The cut is forgiving through the thigh and knee but it still needs the waist and length right, otherwise the whole point of the shape gets lost in the fit.
Before you buy, check where the waistband actually sits when you're standing normally, not pulled up for the mirror, and look at the hem against your actual shoes, not bare feet, because an inch makes all the difference with a straight leg. Check the fabric isn't see through when you stretch it across your hand, the pockets lie flat rather than gaping, and there's a bit of structure at the waist, a proper button and a fly rather than just elastic pretending to be tailoring. Sit down in the changing room too. If they tighten across the thigh when you bend, they'll do that all day at a desk or on a bus.
A well made pair feels like something between stiff and soft, cloth with a bit of body to it so the leg hangs straight rather than clinging or swinging about. Run your fingers over the inside seams, they should be finished, not raw thread that'll fray after two washes. The hardware on the waistband should feel solid rather than light and plasticky, and the fabric should have a slight weight to it, enough that it doesn't crease the second you sit down. Cheap ones usually give themselves away at the knee first, a shine or a baggy stretch that never goes back, so run your hand there and imagine it after a full day of wear.
If you've got none, start with a mid weight pair in a fabric with a touch of stretch, enough give to sit through a full shift without digging in, but enough structure to still look like trousers at the end of the day rather than leggings that gave up. A mid rise sits comfortably for most body shapes and doesn't dig in when you sit for long stretches, which matters more than people admit until they're three hours into something and shifting in their seat.
In an ordinary wardrobe they pair with almost anything you're already reaching for, a simple jumper and flat boots for most days, a plain shirt tucked in when you want to look a bit more finished, even a long cardigan over the top for extra length and coverage on a cold morning. They don't ask much of what's around them, which is exactly why they end up on repeat.
Start with black if you want one pair that works everywhere, but if you already live in black trousers, try a soft navy instead. It does the same job without looking like you own the same thing twice.