1. Volatility — how much risk do you want?
Volatility refers to how often and how large a slot pays out. Low-volatility slots deliver small wins frequently — your budget lasts longer. High-volatility slots may go a long time without paying, then suddenly produce a big win.
If you want to play longer on a limited budget, choose low volatility. If you are looking for the chance of a big win, choose high volatility.