The 2 lines I have here are Zen Internet (which runs the forum, as well as other stuff), and BT Connect for Business, which is work provided.
Both are only as good as the lines they run over, but my lines are some of the best on the estate

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The Zen line in particular is very stable - I've had line uptimes of over 400 days (helped as its a legacy product), and always get the full bandwidth that the line suggests (again, helped being a legacy product - uses a different VP on exchange backhaul). It has a 100Gb monthly download limit (Zen don't count uploads like every other ISP), and has proven ideal for my advanced needs. Expensive though.
The BT Connect slows down at peak times - I think this is exchange contention. Has unlimited downloads (suspect a FUP in there somewhere - but I transfer a huge amount of data over it during the day for work).
What do you want to use it for?