This is a great question, that we get a lot. The answer is all versions. Our hosting is not locked down at all and it’s not SAAS. Each hosting account with us is completely separate and private just to its owners and we provide you with full access to your files and databases. When we install CiviCRM for you, we install the latest stable release (unless you request something different) and then after that, we do not automatically run upgrades.
You can always contact us for upgrades, but anyhow if you don’t, we will not update your software for you.