Welcome to FT!

None of the programs use "sliding windows" qualification periods. It is either from January to December like Miles and More is doing it or 12 months from the month you joined the program like EuroBonus is doing it.

In a lot of cases sliding windows wouldn't be too great... you would need to have a pretty constant pattern of traveling to maintain status.

As usual, it makes most sense to be member in the program of the airline you fly the most or serves the region you have to go a lot best.

For me EuroBonus works pretty well while e.g. at Miles and More I probably wouldn't even qualify for *G.

Also, what do you expect from the program? Easy award redemption at low cost? Then SK is in fact a good program, probably better would be UA Mileage Plus. Just status for lounge access? Then perhaps TK or A3 will be easier for you to achieve status. LH/LX first class awards? Then you need Miles and More or a lot of patience