As has been said, there isn't really a best, just what you like the most. I've found Linux Mint to work really well as a beginner Linux OS for dipping my toes in the big Linux pool. I even managed to convince a friend recently who isn't great with computers to try it out and so far she seems to like it.
The few problems I've had with Mint were resolved fairly easily with some googling and very light tinkering (pretty much just copying and pasting a fix into a notepad file). So far, it's been going great.
While I didn't mind Ubuntu too much, and it definitely has a nice fancy/polished look about it, I found it wasn't for me. Like many others, I wasn't a fan of the desktop environment, and it seemed a bit bulkier and possibly (although I didn't run any real tests so it could have just been my imagination), slightly slower than Mint on the laptop I was trying it on
If you do choose Mint, and it asks you what type (desktop environment you want) out of Cinammon, KDE, MATE, and Xfce, I would recommend MATE right now. Initially I had Cinammon, and while I really liked the layout and feel of it, it was a big resource hog that would randomly spike up usage for no noticeable reason. Mate looks, to me, somewhat similar with a somewhat similar layout, but with no random resource hogging.
Depending on your hardware, as in if you are using a relatively low powered computer/laptop, Lubuntu could be an option. It isn't as full featured, but it also doesn't use many resources and runs surprisingly well on older/lower-powered hardware