Montezuma - Gifts for the Tlatoani [24 - 3 = 21] It's a good ability, which works well with everything else. You are encouraged to expand and gain control over as many luxuries as possible. As you do, you get stronger, and the added amenities help you keep your large empire happy. That said, neither part of the bonus is that powerful. Luxuries are geographically bound, so by the time the combat bonus becomes really significant, you will probably already have a large empire, and be on your way to victory anyway. The bonus will help you win faster when you are already winning.
Qin Shi Huang - The First Emperor [23 + 1 = 24] I'm somewhat torn between Qin and Trajan for my favorite, but Qin has received too many downvotes for having "just 1 extra builder charge, when you can get +3 from other sources". Thing is, you get this +1 right from the start, when it matters the most, and you don't have to spend policy slots or governor promotions to get it. Civilization has always been a game of snowballing, and that early boost to your builders is a significant early advantage. I also just really like this ability thematically, as it makes China a great builder of early wonders.
Tomyris - Killer of Cyrus [26]
Trajan - Trajan's Column [32]
EDIT: Added in Zdarg's missing votes.