It depends hugely on personal taste really...
John Lewis (their own brand) is by far my favourite. Good quality, not wildly expensive, wear very well, and they have nice unusual prints and colours that are totally suitable for either boys or girls. Their multipacks of vests are brilliant, with nice and bright fun colours. Recently my little boy has had some lovely JL bits - a fab hoodie with dragons and squirrels on, a jumper in a lovely jungle print, and various bits with llamas / cactuses on.
I also quite like Sainsbury's - if you sift through the stuff with slogans you can find some nice bits. They're quite good for jeans / joggers / basics. We got some fab joggers with orange dinosaurs on them the other day.
H&M also quite good, although the sizes are confusing.
I loved Boots when my boy was tiny - they have such lovely babygros and rompers in bright unusual prints. Often on 3 for 2 as well.
I do like the look of Frugi and those more expensive brands, but they're overpriced so tend to only buy in the sale. Or my mum tends to splash out on some more expensive bits now and then.
Blade and Rose leggings are gorgeous and are so stretchy that they last and last so good value.
For independent clothes makers you can't beat Boy & Bird if you want something special / custom.