You are in a plumbing and heating forum. You will get replies here from homeowners, DIYers, plumbers who fit maybe 6 boilers a year, repairers, and finally installers who fit over 50-100 per year.
I'm in the latter category and I happen to fit just Vaillant, Bosch and Viessmann, nothing else in the domestic small gas market.
I know which models give us the most trouble over the first 5-7 years, but if I say here, lots of the former people will vehemently tell me I'm wrong.
If you REALLY want to know which manufacturer is best, ring their service helpline this morning and see how quick they answer (numbers are on their web sites). I refer to the OP, dont all do it, it will waste valuable resources!
One will stand out from the other two. If you get cut off or no answer tell us who it was.
The one with the customer service that stinks will surprise those that read this thread like it was the gospel, because I know what the answer will be.