I've used all 3. I was an original beta tester of both Parallels and Fusion. I tried Virtual Box for about 3 months before abandoning it. Here's my take:
VMWare Fusion - Most stable. I'd choose this for primarily productivity applications and light gaming. Word, IE, Visual Studio. if I need it to work, that's my go to.
Parallels - Fastest. Parallels, in my experience is the fastest of the bunch. It sometimes seems they achieve that by taking shortcuts with quality. It's not unstable, but it's not rock solid either. If I was primarily using it for game, browser, or other activities that didn't matter if they bombed, I'd pick Parallels.
Virtual Box - I'm pretty technically savvy. I spent 3 months experimenting with Virtual Box and eventually got everything working correctly. (It was working day, but getting everything working the way I wanted took a bit of twidling.) But it was way more effort than casual users are likely to endure. It's not bad (in fact it's quite good considering the price) but given that you can typically find both Parallels and Fusion on sale for $20-40, I'd choose one of the other 2. Free is great, but in this case I think the other 2 products more than make up for their cost relative to VBox.
All IMO, of course.