I tried avoiding Amazon for a while, but went back to using them. They might use lots of tax loopholes and treat their employees badly, but from a customer service point of view, they are much better than smaller (and some large) competitors. Accurate stock levels on the website, fully tracked cheap fast delivery, big range of products, hassle-free returns.
When I buy from Amazon direct (not some marketplace sellers) I can be confident that the item will turn up on time. Unlike smaller retailers, where it feels like you're taking a gamble nowadays - will they dispatch it today, or just wait 2 days? Will they use Royal Mail (hopelessly slow here at the moment)? Will it be a load of hassle to get a return or refund it if arrives broken? So whilst my heart wants to avoid Amazon, I often use them if there is no other vendor that I haven't used before and trust. Smaller retailers need to get much better before that will change.
Large companies I avoid:
Royal Mail: Since COVID, their delivery times seem to have gone up to about 1-2 weeks here. Sometimes up to 6 weeks. Not that useful to have one's credit card statements arrive after the due date. I'm using Hermes for things I care about arriving on time, until they sort this out, and am avoiding online sellers who only offer RM.
Currys/PC World: Every interaction with them has been a total nightmare. Hard pressure selling for upgrades, insurance etc. Appalling stock control (if something is "in stock" on the website, don't take my money and then declare it out of stock), unknowledgable staff who often flat out lie to secure an upsell
Vodafone: Lying salespeople who sign you up to contracts you didn't agree to, and customer services who couldn't care less.
EE: Changed my partner's Orange contract unilaterally in their favour when Orange and T-Mobile merged and refused to accept the old terms. Had to threaten court action after they sold the "debt" for this fictitious new contract and refused to back down.
Tesco: Their fruit and veg goes off real quick. Whilst staff at some stores are good, some stores have awful staff and management who have zero customer service skills.
Any small electricity or gas supplier: They all seem to go bust and then you get trapped on an expensive contract with your money in limbo for weeks whilst Ofgem transfer you to whoever the new deemed supplier is.