There are more secure OS's, but there's no such thing as 100% secure. Humans made operating systems and they can unmake them. That being said, Linux and other Unix based OS's are generally more secure than Windows, Mac, etc. mainly because they are less popular targets, but also because they are generally open source. You do have to be a power user to find a distribution (Ubuntu is the one of the most popular), install and maintain it though. You can run it on most computers out there, but you have to be willing to get all drivers for it. That's not always easy because some hardware might not have a Linux driver.
You can install Linux on tablets (android and ios), but that's quite the effort. Naturally, you're voiding your warranty if you go that route. Again, you run into various software issues when trying to install Linux on a tablet or smartphone. Not impossible, but very involved.
