Australia - Land Down Under [20]
Greece - Plato's Republic [20] [19+1=20] Others have said it's useful only early, but I find there are some powerful cards you can run later in the game that can produce some significant yields. Colonial taxes is one of them (if you are split between 2 continents). Often I don't have room to slot this in, but with this ability I can. Rationalism, Grand Opera, Simultaneum are all useful abilities you may not have room to run without this ability, and obviously help you with various victory methods. These cards will produce significant yields (with the appropriate infrastructure of course)
Japan - Meiji Restoration [13]
Kongo - Nkisi [9]
Nubia - Ta-Seti [3] [6-3=3] Very awesome in the early game during (and before) your initial archer rush. But after that? Do you really need to build more archers? Unless you are careless with your uu, no. The promotions are useful, but unless you are going full on domination (in which case cavalry units will take over in usefulness), they aren't necessary. I like something with a little more staying power. The mines bonus is useful, but doesn't apply to all mines.