Domain — Look at the top X cards of your library, where X is the number of basic land types among lands you control. Put one of those cards into your hand and the rest on the bottom of your library in any order.
To determine the number of basic land types among lands you control, look at the lands you have on the battlefield and ask yourself whether the subtypes Plains, Island, Swamp, Mountain, and Forest appear within that group. The number of times you say yes (topping out at five) tells you how powerful your domain abilities will be.