No genealogical or family history source is 100% reliable.
The originals of official records (e.g. photographic images of the actual handwritten birth, marriage and death records, census records etc, whether these are on the state government site or on a commercial family history site like Ancestry or FamilySearch) are the most reliable records. But even they are not always 100% accurate. People lie in census returns all the time (e.g. they may claim a live-in lover is their wife or pretend to be younger than they are). They sometimes lie in official birth/marriage/death certificates (e.g. although this is in the British records, not the US ones, one of my relations lied about her daughter's birth date when registering her birth, probably because she was late registering the birth and wanted to avoid a fine.) And mistakes can be made, either by the family member or by the registrar (e.g. a child registering their parent's death may not actually know their exact age or place of birth and may provide inaccurate details e.g.2 a registrar may mishear what the family member says and write down the wrong information).
Transcriptions of original records (e.g. those on Find A Grave and those records on Family Search where you have no chance to view the original image) provide further opportunities for mistakes to creep in (e.g. transcription errors, where the person/computer copying the record misreads the handwriting and types Brown instead of Brain etc, and ) and ideally you should only use them as a guide and check against the original document when you can. If the original document is held in a foreign country or distant state and has not been uploaded onto the 'net, though, sometimes a transcription is the best you can get hold of for the moment. You should really only consider it provisional information, rather than reliably proved information, though. Transcriptions on a reputable site, staffed by experienced genealogists, are usually more trustworthy than ones on private websites or on user-submitted websites like FindAGrave where anyone can contribute content without any quality control.
Obituaries, being contemporary sources, have some merit, but are only as reliable as the person who provided the newspaper with the info - the daughter of the deceased may think her father was born in Lackawanna County, but as this happened 20+ years before she was born, she wasn't there to witness it and may be wrong about that. They are not as reliable as official records and, in addition, often contain typos and other editorial errors which can really mislead you.
Generally the least reliable sources (in fact, I wouldn't call them sources at all) are user-uploaded family trees on Ancestry.com, Rootsweb World Connect and other such websites. Many amateur genealogists make mistakes in their research and some of the trees I've seen on these sites are works of pure fiction.
That's not to say they don't have their uses - if I think I've found a match with my tree and the submitter looks to be an experienced genealogist who knows what they're doing, I will often PM them and suggest we share information and sources. Often they've found records I haven't and vice versa and sometimes a seemingly unsolvable mystery can be cracked by pooling resources and using two heads instead of one. But I will never take information in a user-submitted tree on trust and will always go back to the original sources they've used and check them myself. I never simply copy and paste information from other people's trees.
It's usually best to use as many sources as you can, so you get as complete a picture as possible. None of the sources you've listed are actually bad, with the possible exception of user-submitted family trees which, like I said, are not a source.
The other issue is people using sources badly - a common mistake which many amateur genealogists make (and which I made myself when I first started out) is to assume that if an ancestor called Adelaide Jeffs was married in one town, and there was also an Adelaide Jeffs of about the same age born or baptised in that town, then they must be one and the same person. But people move about and don't necessarily get married in the same church/town/county/state as they were baptised/born. You can make this mistake even if the sources you are using are original records and 100% reliable. Your research technique is an even more important factor for the accuracy of your research than what sources you are using.