For all its faults, Ancestry redeems itself somewhat by having a very powerful search engine, so you can often find things in spite of the sometimes bizarre transcriptions.
The other thing you need to watch out for with Ancestry is that different databases are indexed in different ways, even when the are superficially similar. The documents in the two WW1 Army Records series are very similar, but the Service Records (aka Burnt Documents) includes a searchable field for residence that the Pensions (Unburnt Documents) does not have.
Some of its databases, like the census, have been indexed by/for Ancestry, from the original documents. Others, like the telephone directories, have been scanned using OCR, and are 'untouched by human hand' as it were. Some of the results are a little odd!
Findmypast is generally good with regard to quality of indexing - their very first product was the full set of BMD indexes, which were orginally produced for in-house use by Title Research, the parent company. So they had a vested interest in their accuracy. Since then they have changed their name from 1837online and produced more databases, as well as acquiring existing ones from a business called The National Archivist, and from Familyhistoryonline. I can't vouch for the former, but the Familyhistoryonline material is generally of high quality. My main criticism of Findmypast is that their website is not as user-friendly as it might be.
The Genealogist generally has good quality information and some useful features, but it is very poor at describing the actual contents and coverage of a database. Their BMDregisters site is particularly disappointing in this regard, where the descriptions are inaccurate and rather misleading, although it includes clear images of the actual records. It is also very expensive, and the search facility leaves a lot to be desired, but hopefully this will improve as more records are added.
Some actual databases are available on more than one site, but even though the information is identical, the type of search engine used can make a difference. The FreeBMD database is also available on Ancestry, and I generally use the original FreeBMD, but sometimes the Ancestry one is more useful - you get the results in alphabetical order, not chronological as on FreeBMD, and you can cross-search with other Ancestry databases. The original 1881 census, compiled by volunteers under the direction of the LDS Church, is available all over the place, but again, there are differences depending on the search engine. The version on Ancestry has the added advantage that it contains submitted corrections and alternate names.
Finally no-one has mentioned Familyrelatives.com, which I have not used, but I know some experienced researchers who rate it highly.
Mean_genie