White company, Next, Hatley, Mothercare, Cath Kidston and Boden have best quality (and cutest prints) pjs.
I know they're a bit pricier than Primark and Asda but the quality makes up for that. I only say this as someone who learnt from the mistake of buying cheaper pjs which I had to replace after not very long and so spent the same amount as I would have on the pricier ones anyway. Logos on cheaper pjs rub off or crack badly after a couple of washes, they go all bitty and rough, get holes in them, lose buttons, shrink and zips on onesies last a week if that. Also they have no resale value.
You can get them on ebay for quite cheap and still in good condition as well.