Buy bitcoin, that's the safest and most secured coin you can invest right now. Make it a habit also to read and research about the coins you want to invest in, of course people would want to ask opinion on others but at the end of the day it is still you who decides which coin to invest in, right?
The optimal timeframe for which you should count on long-term investment is from 1 to 2 years. It is worth bearing in mind that during this period the price can repeatedly change its direction, so that the main partners of the investor are calmness and internal discipline.