The best CMS is the one that works for the problem you're trying to solve. So, in essence, it really depends on the project. Just going with what's the most popular isn't the right approach. In my opinion, the most popular CMS platforms are starting to show signs of their age (ie. WordPress, Drupal, etc.). There are more modern platforms that solve problems more effectively by allowing for a more modular approach while give designers and developers far more control over the design.
That said, I have two CMS platforms that I currently use as my go-to's:
SilverStripe - A PHP-based framework that allows for scaffolding out a full data model (very similar to how Ruby on Rails works). Allows for complete freedom over the design thanks to a simplified templating model.
Statamic - A flat-file CMS (no database!) that is incredibly fast and easy to work with. Designers like it cause they don't have to know any code to build complex sites.
I tend to use SilverStripe for projects that have a lot of relational data requirements, that is where there is a lot of relationships between various bits of information. Basically, anytime where the content requirements are rather complex. SilverStripe makes it much easier to create sites with complex requirements while still making it easy for content editors to manage the content.
Statamic to me is a WordPress killer. I use it primarily for sites that would otherwise end up as WordPress sites. That said, you can do a whole lot more out of the box with Statamic than you can with WordPress. It's also much, much easier to design with thanks to a very easy to use templating system.
I'm also looking at adopting October at some point. It's built off of Laravel, which is a very modern PHP framework. My understanding is that October is highly modular and allows for a ton of flexibility under the hood. Might be good for a project or two.