I can't think of a single other country that claims to be civilised that has a tax code so complicated you need vast amounts of software and a high-power computer just to file what is properly owed.
TLDR version: The system is engineered to be too complex for humans, which is the mark of a very very badly designed system that is suboptimal, inefficient, expensive, and useless.
Let's pretend for a moment that you've a tax system that taxes the nth dollar at the nth point along a particular curve. We can argue about which curve is approporiate some other time, my own opinion is that the more you earn, the more tax you should pay on what you earn. However, not everyone agrees with that, so let's keep it nice and generic and say that it's "some curve" (which Libertarians can define as a straight line if they absolutely want). You now don't have to adjust anything, ever. The employer notifies the IRS that $X was earned, the computer their end performs a definite integral between N (the top of the curve at the last point you paid tax) and N+X, and informs the employer that N+X is the money owed for that interval.
Nobody actually does it this way, at the moment, but that's beside the point. We need to be able to define what the minimum necessary level of complexity is before we can identify how far we are from it. The above amount has no exemptions, but honestly, trying to coerce people to spend money in particular ways isn't particuarly effective, especially if you then need a computer to work through the form because you can't understand what behaviours would actually influence the tax. If nobody (other than the very rich) have the time, energy, or motivation to find out how they're supposed to be being guided, then they're effectively unguided and you're better off with a simple system that simply taxes less in the early amounts.
This, then, is as simple as a tax system can get - one calculation per amount earned, with no forms and no tax software needed.
It does mean that, for middle-income and above, the paycheck will vary with time, but if you know how much you're going to earn in a year then you know what each paycheck will have in it. This requires a small Excel macro to calculate, not an expensive software package that mysteriously needs updating continuously, and if you're any good at money management, then it really really doesn't matter. If you aren't, then it still doesn't matter, because you'd still not cope with the existing system anyway.
In practice, it's not likely any country would actually implement a system this simple, because the rich would complain like anything and it's hard to win elections if the rich are paying your opponent and not you. But we now have a metric.
The UK system, which doesn't require the filling out of vast numbers of forms, is not quite this level of simple, but it's not horribly complicated. The difference between theoretical and actual is not great, but it's tolerable. If anyone wants to use the theoretical and derive an actual score for the UK system, they're welcome to do so. I'd be interested to see it.
The US, who left the UK for tax reasons (or was that Hotblack Desiato, I get them confused) has a much much more complex system. I'd say needlessly complicated, but it's fairly obvious it's complicated precisely to make those who are money-stressed and time-stressed pay more than they technically owe, and those who are rich and can afford accountants for other reasons pay less. Again, if anyone wants to produce a score, I'd be interested to see it.