It’s hard to top dropbox for the feature set and space it offers. Especially now that it has the smart sync options as @trickyflemming mentioned. I use it for all personal and work files. It’s excellent for collaboration since you can easily share existing or create new shared folders with just about anyone because it has a good free tier offering.
However there are a few caveats. Firstly, never use it for tracking lots (multiple thousands+) of small files that change often. If your dropbox reaches 500k files, it will becomes very slow. Its indexing performance, in my experience, has been incredibly slow and resource demanding (despite SSD on a MacbookPro). This isn’t an issue 95% of the time because it usually only has to upload/index a handful of files at a time, but that 5% can be frustrating if you’re in the middle of something.
Secondly, some people have expressed privacy and security concerns due to Condoleeza Rice being on the board of directors as a result of her role in started the Iraq war and authorization of warrantless wiretaps. The campaign (2014) to oust her failed and there hasn’t been much talk since. Unfortunately for me, due to work, I don’t have much choice in the matter and I suspect that is commonplace. To be fair, I’ve not heard any reports of wrongdoing so far.