Some good advice has already been given. Different threads have different properties. As your experience grows you will get to a point where you will be able to pick a thread for its properties and how they suit what you are tying.
What we need is a sizing system for thread.One which we and all the thread makes stick to. Some don't even stick to the sizing method they use. Denier would be a good idea but, as it is based on the weight of a given length of a thread is useless for comparisons across ranges. Say silk thread to polyester thread. They will be of such different sizes for the same denier that the system is useless. Even worse is the aught system. "0" Was the finest thread which could be spun from silk. Time passed, spinning techniques developed, and the thread size shrank. When it got to half the size it was they called that "00". Each time the thread size halved they added a "0". Therefore, 7/0 (0000000) would be half the size of 6/0 (000000), 8/0 would be half the size of 7/0, therefore, 8/0 would be one quarter the size of 6/0 Is it??? So which Uni thread is correct to the "0" system? We just don't know. Even worse is that one maker's 8/0 is another's 14/0. To add to the confusion Uni may be 136 denier, that means the weight, not the size, of the thread is similar to UTC 140. Its a nightmare.
Trying to sort this out is not easy. You could state the diameter of thread, but that varies with twist, how much twist over what length, and what about the threads that can not be completely flattened before putting the right number of twists in in to measure?
All I do is to think about the techniques I will use, and select a thread who's properties suit those techniques. If I am asked what thread I am using I can state it and why. That doesn't mean other threads will not do just as good a job.
Thread isn't cheap, sorry Bugsy, we buy it is too small quantities for it to be cheep. Several years ago I bought a 15 000m spool of bonded polyester thread that was a little finer than Uni 8/0. It cost me about £9. At that time a 200m spool of uni cost, at my local tackle shop, £2.20. Which means to buy the same length of Uni would cost £165. By buying thread in such small quantities you loose all economy of scale. When you buy a spool of thread you are paying more for the spool than the thread which is on it. If you know a lot of tiers, or are in a club, then I suggest you get those interested together. Each buys a spool of 1 colour of Guttermann Skala. Repurpose a sewing machine motor as a spool winding motor, and save yourselves a small fortune.