As you see from the responses above, there really isn't one solid answer. I don't think any ride breaks all the time or consistently (yes, I know FOP had issues when it first opened, but I'm talking long term patterns). Some are weather dependent, like TT. But over the years, I've experienced a ton of rides that go down from time to time. Got stuck on Pirates once and had to be evacuated. So I think the real answer is "it's random".