stable version of BIG-IP
what is the stable version of BIG-IP ? 15.x , 16.x or 17.x ?
do you recommand to upgrade to last version?
It depends, which modules of F5 do you have provisioned? every version of F5 has bugs to discover depending on your modules and configuration, so my advice is to review the Release notes of each version or wait for others to other customers to discover the bugs in a version, for example, Version 17 that was released two weeks ago.
But if you want to use a new feature in the last version you can always update one of the devices in your cluster and wait for a week to update the second and discover if your configuration gets a bug.
I have a lot of customers in the last, intermediate, and old versions, and everyone works well, but in some cases is necessary to roll back the last version for a bug.
Take in mind that there is not a definite version that solves every issue and in the current world, the security advanced every day, and updating the systems is really necessary to close bugs and vulnerabilities.
Creating and following an updates plan is really necessary to keep your F5 safe.
Sebastiansierra's answer is a good one. I'd add that you should always weigh, in addition to features needed and bugs present, the clock on releases regarding end of software development and end of technical support as covered in K5903. If you're trying to decide between 15.1.x and 16.1.x, and bugs are not a concern, I'd recommend 16.1.x as both 15.1.x and 16.1.x are stable maintenance releases, but you get more features out of 16.1.x AND more time to move forward to the next stable release once available.