It's not necessarily about spending the most money - it's about buying the quality that you need.
I like 100% cotton sheets with a thread count of 400 - more than that makes them a bugger to launder so even though they feel lovely, I prefer the lower thread count because it makes life a bit easier.
Dog food - I don't buy the most expensive brand out there, but the stuff we use is not cheap by any means. Worth it though as it solved multiple digestive issues! Cheap dog food can be a complete false economy if your dog is constantly at the vet with various digestion problems.
Shoes - you spend a lot of time in them! The best pair I've bought recently were some Sketchers. Memory foam insoles, very supportive, ideal for hoofing between tube stations in a rush. They were only £35 so you don't necessarily have to spend 3 figures.
Skincare - I use a lot of Korean and Japanese products. One thing I will spend money on is sunscreen - the Asian SPFs are hands-down better than what you can get here.
Food - I buy organic milk and free range eggs. I'd rather have less of them than buy cheaper. Meat is of the highest welfare standard I can find and afford, although I am trying to cut down on how much meat we eat.
Chocolate - although it's a very occasional treat as I am trying to lose weight. I don't like dark chocolate but am also trying to avoid palm oil. Godiva is lovely and worth the money I think.
Bras - if you have large norks then good quality bras are essential. Mine come from Bravissimo but I try and stock up in the sales to save money.
Things I have economised on -
Own brand cereal - Tesco rice krispies, shreddies and special K are just as good and half the cost of the branded equivalents.
Dishwasher tabs - Sainsbury's own have exactly the same ingrediants as Fairy Platinum and having given them a go, I'm sticking with them as they work really well.
Shampoo & conditioner - if you like the Aussie stuff then go to Aldi as their versions are really good and much cheaper. I actually prefer the smell of the Aldi ones!
Leggings - best ones I have found are from Sainsburys (the soft touch luxury ones). Wash well, don't bag at the knees and they don't bobble either.
Pants - if you aren't bothered about matching sets then Asda multi-packs of cotton pants are good. They wash well and keep their shape and are very comfortable. I used to buy Sloggi but wouldn't bother again - and the Asda ones are better than the M&S ones I was also wearing.
Hand cream - Nivea soft. It's better than any other I've tried including some of the really expensive ones. Cheap, non greasy and works really well.
Baking supplies - from Aldi or Lidl. I bake a lot and the price difference for flour, sugar etc. is noticeable and the quality is fine.