In the UK it is honestly 6 to 1, half dozen to the other.
I've been on Three, Voda, EE, O2. All of them are crap in London. All of them are crap where I live. None of them were good from a billing perspective (actually with the exception here being Three!). Voda even double billed me for 3 months before I noticed it and had to request a refund.
Conversely, I went to New York for a business trip and 5G was actually good there. London is utterly awful for any of the frequencies - 3G was decent before it was over-saturated, same with 4G. 5G started off rubbish and has gotten worse due to the ridiculously short range and number of masts required (which you just cant get approval for in London, hence why its so bad).
Try getting a mobile signal anywhere around London Bridge. Exact same issue - over saturation, 5-bar signal, no throughput whatsoever. Thats on 3G, 4G and 5G.
Three were genuinely (from my anecdotal experience at least) the least worst of all of the mobile operators.