Favorite Civilization V Civilization (DLC's included)

So as a strategy fan I find myself usually playing every strategy game reviewed by IGN that my friends recommend, and lately, I've been playing Civilization V, and am loving it.
Personally I like to expand early, and stick with what I have, so the Shoshone are great, I also like Babylon because of their science boosts, you can easily rush writing and then take off pretty much. I'm only now experimenting with tourism, but I like Brazil the most for that kind of victory. Lastly, I really enjoy Rome for domination, you get iron fast and get ballistas, and you'll be able to defend or attack effectively for the next era. So what about you guys?

I really need to play Civilization 5, for some reason I skipped it. Are the expansions a must-have or is the vanilla game good on it's own?

It's funny because for the amount of Civilization V I play, rarely will I finish a game. Usually at the beginning of the Atomic era is where my boredom sets in to which I'll start a new game. I'm not alone on this apparently.

For science victories, the best I have found are Korea and Babylon. Though out of the two I'd always choose Babylon; quick great scientists, along with the early one for writing, can put you in front by a wide margin.