Which is the best strategy game out of these?

Crusader Kings 2 is the more involved of the two. EUIV is a blob simulator that is significantly casualized from III. If you don't mind the incessant overpriced DLC, get CK2. The game and its DLC are on sale on both Steam and Amazon for hefty discounts, so now is a good time to pick it up. Just make sure you actually care about all the DLC before you buy it all.