Gradients can not be defined as "best" or "worst" as they depend on many factors. For example, if you have a layout room 20 feet long then a gentle gradient of, say, 1 in 60 will allow a line to climb enough to cross over another, but in a smaller room it may have to be 1 in 30 to obtain the same clearance. If it is solely for scenic purposes then around 1 in 50 would probably be the maximum you should aim for.
Another factor is what type of line, and train, do you intend to run. On a main line with 12-14 coach (or 70 -80 wagon) trains behind a steam loco or early diesel then it may have to be no more than 1 in 80 - 100 unless you intend to reproduce prototype situations with banking engines. This would be a very interesting exercise, allow for large numbers of locomotives in a prototypical setting and easy to reproduce with DCC control. On the other hand, in a depot with lots of light engine moves, it could be as steep as 1 in 30 although that is about the maximum you should consider to look real.
One thing you should not forget is that if you are raising one line just to cross another then if the lower line falls at the same time the gradients can be substantially reduced. If a loco starts to slip then either the line is too steep, the load too heavy or both.
One last point, there are any number of good books available covering this subject, along with baseboard construction, and it may be best to consult them before starting work.
In short there are no easy answers to your question as there are as many solutions as there are questions.
I'm not sure if I have answered you question but I hope so, I've done my best with a limited idea of what you really want.
Good luck, anyway.
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