TBH, I find High st shopping pretty depressing these days: there seems to be little difference in quality and vast differences in price.
I do find Gap excellent for their cotton (after a blip of a few years where they must have been using inferior cotton), and have had a lot of success with their knitwear over the last few years: not very bobbly, good cut and designs and wash well.
I like Banana Republic for work trousers, and would buy more but find they're a little too conservative for my tastes. Some fantastic silk blouses and the like, and their sales are excellent.
I now find myself buying more and more from Me&Em online. Good sales, and excellent quality fabrics. Unfortunately (for me) their reputation seems to be gaining so much ground, often their stock sells out before I can get my grubby hands on it.
Denim I tend to favour premium: I wear my jeans a lot, so I pay more (although still look online and in sales as there are plenty of bargains to be had). This gives me a better cut (less hitching up through out the day, softer denim so no rubbing, better wash effects to flatter etc etc). On denim though I don't think there is a definitive best brand, it's all done to trying hundreds of pairs on until you find the right pair.
I'm still wearing Primark strappy vests from a couple of summers ago, and they have washed brilliantly. Nothing else from Primark has lasted. I have a few Baukjen long Ts for layering, and they have also been brilliant (buy in sales). I've yet to try much Hush stuff, but bought some cashmere last year that has worn very well, so I'll likely go back to them for more as and when I need it.
My biggest issue is footwear: I don't buy loads of shoes, so am happy to spend more on what I do buy, but demand comfort with style, and that's so so hard to find. Still on the hunt for the perfect pair of ankle boots...