Different OS platforms offer advantages and disadvantages in term of security as far as user is concerned. Some maybe open-source, others may not be. Some may have huge user base, others may have a moderate user base. Some maybe more and others maybe less leaky than others. Some maybe more tamper resistant and some less tamper resistance than others. Some maybe more resistant to malware and other maybe less resistant to malware and so on. What is the preferred OS platform with respect to TBB in the sense of minimization of security risks?
Even though Tails is without doubt the most secure OS you can get for Tor, it may not be suitable for you in every senario. One reason is because it is a live system, not an installed system.
If you want an installed system, most larger Linux distributions is probably also safe (e.g. Debian, Fedora). The exception is Ubuntu, which is very leaky by default.
To not upset the FreeBSD/OpenBSD users, those is probably also safe.
For the paranoid:
The unfortunate truth is that there does not exist any secure operating systems. There does not exist any way to prove that an OS does not contain any security issues. There does not exist any way to prove that an OS is not backdoored. And the situation is worse that that; not only does there not exist any method to prove they are secure, they are not secure. Every week new vulnerabilities are found, which possibly can be used to compromise your system without you knowing it. Even if you update your OS every day (which you should), it will not be secure.
All operating systems for end-users out there can only be put in two categories: Low security, and totally unsecure.
Which OSes do you (or your helpers, paid or free) know how to administer most safely (fixing security bugs, making sure configuration is safe)? And which of those has a currently maintained Tor Browser Bundle available for it? As a first cut, use that one.
For a second cut, look at Threat Event Frequency estimates. How many Threat Agents are there in the world potentially interested in you who are also skilled in breaking a particular OS or its applications? How likely are they to come after your systems? Then, how easy will it be for them to get into your systems when they do pay attention to you?
The general population of Threat Agents typically go after bank accounts, and their attack frequency is fairly high. There are many, many Threat Agents skilled at attacking MS Windows (despite MS Windows skilled engineers making it objectively harder to break into in the last 5-8 years). There are significantly fewer skilled at attacking Mac OS X and Linux distributions, and even fewer capable of doing something on FreeBSD (including Dragonfly, PCBSD, ..), NetBSD, and OpenBSD.
A less general population of Threat Agents will go after people performing activities they don't like, generally speaking. These agents includes everything from well funded government employees (spies, or law enforcement) to patriotic hackers and similar activists up to obsessed griefers. There is a more level spread of skills in various OSes in this population, though MS Windows is still generally the best understood.
For Tor specifically, if you consider yourself at highest risk from the general Threat Agent population described above, I suggest Tails, or running TBB under Linux, Mac OS X, or MS Windows (in order).
First, try Tails https://tails.boum.org/ as suggested elsewhere.
Second, I would consider running the Tor Browser Bundle (always latest version) on debian or Fedora Linux (always keeping them updated with patches). Find someone to help you run debian or Fedora Linux safely. Both are actively developed and maintained, so patches will be available relatively quickly for any discovered security problems.
Third, I might try Mac OS X, or after that, Windows. Like above, keep the Tor Browser Bundle and OS aggressively up to date. The reason I would do these as a third choice comes down to Threat Agent skill sets (against MS Windows) and the slightly more opaque configuration of Mac OS X for safety making it harder to find experts to help you.
If you consider a far more specific Threat Agent likely to come after you, you may be able to dodge their skill sets by going for the cutting edge.
For this, perhaps find a way to build the Tor Browser Bundle for OpenBSD armish, and use that (always keeping both at the latest version, patched). This build will not be a trivial task, but it should be achievable. And if you get TBB built for OpenBSD on arm reliably, some others who want the same will be very happy if you share.