If you're going to traditionally publish then you're going to want to follow the guidelines that publishers and/or agents look for. Since I know someone will make a comment along the lines of "But so-and-so published with this company and their book is x amount of words" let me cover that now.
Once again, there are always exceptions to the rule. Maybe they were already an established writer of some kind or they had a connection to the publishing world. Someone like Stephen King, whose name alone will sell the book, can get away with having a book above these guidelines. Someone like me, a nobody with no connections of any kind, won't have any flexibility. Don't go into it expecting to be the exception. It's one thing to do the work and simply not reach one of these goals, then to not even try because others before you have succeeded. Wouldn't you rather give yourself your best shot?
Self publishing won't matter. I know a few people who won't buy a book that is, say, 600 pages because they'll assume the author didn't properly edit but they're the exceptions. Not every reader will care. Only reason publishers care is because they're a stubborn breed. They also have to worry about their printing cost. A book that is 200K words is going to be more expensive to print than one that is only 80K. Thus unless your book absolutely wow's them, you won't have wiggle room.
When I decided to take Freelander seriously and work on cleaning it up, I did a lot of googling to find the word counts that publishers/agents look for. At the time I believe Freelanders rough draft clocked in at something like 140K or 150K. I can't remember exactly and I'm too lazy to open a previous draft to check. For a rough draft, don't care about word count. Just write. Hell for a second draft, don't care. But when you get down to the final draft it's time to care.
Once I found out the average word count for a fantasy newbie author, I began the rewrite with that goal in mind. With the last draft I did before walking away, I got it down to close to 110K. I think it was actually at 113K. While it's still high, it's in a lot more acceptable area.
Your genre 100% defines the word count. A fantasy or SciFi book is going to get a lot more leeway in word count than any other genre. Publishers and agents understand that fantasy requires more descriptions because of the world building involved in them. If your book is over 100K in words, chances are there is some place to cut. Even in a fantasy book.
Fact is, publishers will provide an editor (if they're a good publisher) but they still want to do as little work as possible. Make your story shine before you query.
All right here goes the word count list. I'm only going to list YA and Adult genre fiction books since those are the two I write the most, thus I'm more familiar with them. So if you want the count for a Middle Grade book or another book type, you'll have to google it yourself. These also don't take in account the sub genres or mixing two genres in one, it's just a broad word count per genre. These counts came from several websites and I went with the average between the sites.
Romance: 80K-100K but 100K is definitely on the high end for a traditional romance.
Mysteries/Thriller/Suspense/Paranormal: 70K-90K
Romance: 40K-100K
Fantasy/Scifi: Again, this one gets more leeway because of the mechanics of world building. 90K-100K is perfectly acceptable, but most agents/publishers probably won't balk if you go over. A couple of sites I saw said 110K was a safe point, another said 120K. But I'd say try to get it to no more than 100K if possible.
Horror: 70K-90K
Historical Fiction: This is another one that might get some leeway because of the fact you have to describe an act of history. There are some sites that said 120K was the cut off, but the majority had it ranging from 90K-100K.
General YA Fiction: 50K-80K
New Adult: 60K-85K
I think that covers most of the genres. At least the ones listed on the sites I visited. Be the rule, not the exception. There is a reason most query letters are asked to include word count. It's because they want to know if you've done the work yourself.
For grins and giggles, here are a couple of famous first novels and their word counts.
Harry Potter: 77,508
Twilight: 118,501
The Hobbit: 95,022
City of Bones: 130, 949 though I've seen some places list it at 128K (This is a case of the author having an established fan base from her fan fiction so she was probably given a lot more free-reign on her word count)
Looking For Alaska: 64,033
Divergent: 105,143
Gregor the Overlander: 55,665
So as you can see, with the exception of TMI that went well over, most hovered just above or below 100K.