On a hardtail a new big volume rear tyre makes the ride nicer – but you also always want a grippy front tyre.
I’ve just built a hardtail in the last week or so and I’ve gone for a Maxxis Forekaster 2.6” on the rear. Figure it’s classed as ‘aggressive xc’ so should roll reasonably well but still grip ok. My ride on the weekend was bridleways linked by road sections and some of the mud was pure slop. It did better than the 2.3 dhr2 I’m currently running on my full suss bike which I was surprised about. Planning to just leave it on all year round.
At the front I’ve currently got a 2.6” Magic Mary on there (fork is a boost Pike Ultimate) – but when things dry out a bit it’ll have a 2.5” Maxxis DHF exo max Terra on there that I’ve got. That tyre fits through a non-boost Yari on another bike I’ve got ok – but I think the 2.6” Mary would be pretty tight.