The problem I’ve found is that no site has everything; every site online that lets you list games from a database they manage has missing stuff, often a lot of missing stuff. And while depending on the site it may be possible to add games, that can be difficult, and who knows if they’ll approve them.
So, while I used to use an online list (IGN’s), I gave up on that several years ago and now have a spreadsheet to keep track of my now pretty large collection. It allows me to have everything there and add whatever columns or tabs I want, and while keeping it updated takes some work it’s worth it. Nothing online is anywhere near as good. I’ve got a lot of stuff in the spreadsheet now – columns for things such as the score I give each game, how much I paid for it, accessorites required (light guns and such), etc.; and separate tabs for hardware, for how many games I have on each platform, for toys to life stuff, for total numbers of import games I have, and more.
For an online list, I mostly use GameFAQs now since they have one of the most comprehensive databases, though I never have taken the quite large amount of time adding the several thousand games I own that aren’t in my library there. I may do that someday. Also I put a backup of the spreadsheet online so it isn’t only on my computer.
As for the sites online though, GameFAQs has a quite comprehensive listing of console games. Their computer game listings aren’t as complete, however. For that, Mobygames is definitely the best. Most of the time those two are the only online databases I need, though once in a while other sites are needed to find missing info for my spradsheet, from sites that don’t all have listing functions built in, such as KLOV for arcade games or Game Data Room for Japanese-region release info on classic platforms. That site has exact release info for pretty much everything released in Japan on older formats.
As for sites which focus on listing your collection, they are smaller sites so while I definitely haven’t tried all of them, usually they either seem to be just ‘type it all in yourself’ so I’d rather use a spreadsheet since it’s so much more versatile, or they have a pretty incomplete collection. For instance, I just looked at RF Generation again, and with the very first platform on my list, the 3DO, I found like at least ten games I own that aren’t in their database, mostly Japanese import games. I have 80-something 3DO games, so that’s a lot missing. That’s what I remember that site being like, pretty incomplete. That doesn’t exactly make me want to spend more time adding things there. The ability to mass add games to your library is nice though, sure, but the spotty library will continue to keep me from wanting to use the site.