What's the best mobile internet provider in UK do you think?

I've been with 3 for the last 8-9 years and while it used to be ok and has decently priced unlimited data, lately is getting worse and worse. At moments it takes more than a minute to load a google.com page on 4G.

I've been with Vodafone before and swore never again.

A mate of mine said to avoid O2 at any costs.

I know that Virgin is competing each year with Vodafone for the worse service provided.

Are there any that are decent? I'm based in London so coverage shouldn't be an issue.

Thanks!