Hi I always recommend cloud nine underlay
If you're fitting it on the stairs I would recommend the 9mm and everywhere else I would recommend the 12 mm depends on your budget at the end of the day

I would recommend cloud 9 . 11mm

The best underlay all depends on, the carpet being fitted, the subfloor and local conditions/use.
A professional could only advise once visit your home to recommend

Carpenters "Perfect Living" 9mm it is the same weight as Cloud 9 but a lot less money I retail it for £5.50 sqm

I agree with Matt. The subfloor plays a big part in the choice of underlay. And as other fitters have pointed out cloud 9 is regarded by most as the best branded underlay although even there quality control has dropped considerably in recent times. Delamination being a big problem