I currently have 20 WP installs running on a BlueHost VPS for $60 per moth. I've been with BlueHost for years and have no complaints (call me lucky). Soon, my site count will be up to 50+ WP installs (and will be managed with InfiniteWP). These are all very low traffic niche sites (avg under 100 visits per day, per site).
What I care about most of all is the simple need that the sites stay up. I don't have the speed fetish of most folks (especially for these low-volume sites). Phone and live chat support has been helpful from BlueHost, but that's not a big concern because I will soon be outsourcing all my WP install and server maintenance tasks.
I'd love to get on the WP Engine bandwagon, but it's just too expensive for 50+ installs. Synthesis also appears to be too expensive for 50+ installs. I've also been recommended to use Digital Ocean, who are certainly great on pricing.
With all that said, what host (that I mentioned of any other) would you choose and why?