I don’t think there is any “best” – only different. I have ridden Glentress, Innerleithan, Laggan and now dalbeattie.
Glentress is good because of its variety of trails and sheer amount of them. any level of skill can have fun there and IMO you get really good value of downhill for your climbing.
Innerleithan is more challenging perhaps – but less for less skilled riders and less options.
Laggan is fun for all again – and because the loops are short you can go with a mixed ability group and keep meeting up again – and its black run truly deserves to be called black
Dalbeatie is more natural feeling – I have only been there once and done the red – but there are bits on the trail that are very tricky to be red IMO and no chicken runs but a nice feel to the route – and from the maps it looks like you can vary which bits you ride and make up different loops to suit.
So there is no better or worse – just different and some centres will suit some folk more than others.