I've not ridden regularly for a few years now, so some saddle soreness is to be expected (except for my road bike with it's B17) but my mountain bike feels like I'm sitting astride a fence after not too long, worked out that the saddle is the one that came with a Spec Hardrock in 2003! so it's done some miles over the years. This was brought home by hiring a bike for a few days on holiday, touch sore after the first 12 miles on day one, touch sore starting 19 miles on day two but got comfy and carried on - on my mtb I'm finishing a ride stood up as it's not comfortable sitting down.
new saddle?
what should I be looking for?
Any advice on adjusting angle etc also welcome.
As you're just back after a few years, I'd be tempted to suggest just carrying on for a bit to see if you can break back in to it. But as you have several saddles to compare (and you're happy on a B17), a new saddle for your MTB might well be needed.
But as for which one, that depends on your own posterior anatomy, and I'm pleased to say I've never been close enough to it to judge

What suits one bottom can be very uncomfortable for another one.
Having said that, if you're happy on a B17, which is a saddle I rode on happily for decades, you might have similar anatomy to me. So you might like a Cambium C17, which is what I have on my tourer now and which seems like a natural progression from my old B17. But they're not cheap.
On my MTB I have a Charge Spoon, and I find that fine for 40-50 miles maximum, after which I'm starting to feel uncomfortable - on the Cambium I can go 100+ with no discomfort. The beauty of a Charge Spoon is that it's inexpensive (around £20-ish) and it can be resold for only a small loss if you don't like it.