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For cash or stocks and shares? Tonts has posted a link for cash ISA. For stocks and shares I'd look at Monevator depending on what you want to invest in, how much you want to invest, and whether you already have investments with anyone.
Vanguard is a cheap starter for a fresh ISA as long as you are happy with one of their funds (no app available). iWeb is reasonably priced/good selection but looks very dated and no app. I think Interactive Investor is a good compromise of selection/price/usability, but I have less experience with the app-only stuff like Freetrade etc.
You can get referred to Interactive Investor - I think you get a year free and the referrer gets £200. I can refer you or there's plenty other people on here with ii as well (DM someone your email address).
What's the best isa right now?