What is a good rule-of-thumb floating point comparison method selector?

I'm testing some bits of code, a number which involves computation using floating-point values - often very large numbers of these. I have some generic (C++-templated, but it doesn't really matter for the sake of this question) code which compares my outputs, be they scalar or arrays, against their expected values.

I'm faced with the problem of choosing a precision threshold, at least for the two C/C++ floating-point types float and double - for various functions I'm testing. As is well known, there is no one-size-fits-all with respect to comparing floating-point values, nor a single precision value which fits and computation based solely on the data type: Relative vs. absolute error, numerous operations which may magnify floating-point rounding errors a lot, computations which are supposed to arrive at 0 so you can't really normalize by the expected value, etc.

What is a generally-reasonable approach/algorith/rule-of-thumb to choosing a comparsion method (and equality thresholds) for floating point values?