I personally don't see how people can say that Doc is bottom tier, and people who say otherwise are very wrong. Personally, I think he is just below mid tier, somewhere with the likes of Charizard etc, somewhere where he is now.
However, what gets me is when people try and use the excuse of him being a Mario clone to say that he is good. What I mean by that is, in the past I have seen comments along the lines of "How can somebody with Mario's frame data be that low down, Doc is certainly above mid on that alone". Yes, Doc has a lot of Mario's data, but he lacks a lot of what makes Mario good. He can't perform two aerials in a short hop, he has slower air speed (which means he can't wall of pain with Bair like Mario either), and just generally lacks Mario's best traits, such as great Dthrow (Doc's is much less reliable as it launches up higher), Utilt launches higher, meaning you can't get easy cheap damage off of it, and Uair (Mario's Utilt combo extender) doesn't lead well into many other moves, where as Mario can even perform a stairway to heaven with his. Also, Mario does have more distance on his Up B, so though he has near enough the same frame data as Mario, the properties of the moves are very different. Doc still has his uses, and does a lot of things different to Mario, but when people try and use him being a clone of Mario as an excuse to rank him up, it makes me laugh. Doc is different to the other 2 clones in the fact that there are quite a few differences, so where as Marth and Lucina have only slight differences and Pit and Dark Pit have laughable differences, Mario and Doc's are very different, but it really is true that you can just pick Mario and do nearly everything Doc can but better. It's a shame, because I do like Doc (I used him over Mario in Melee), but ended up switching to Mario after a few months of Doc in this game.