My favorites:
1) Morocco
- Everyone wants to send trade-routes to you because of the UA
- The UI helps make deserts viable for settling or puppet-ing, and in some cases, makes tiles actually better than grassland or plains, even (when built on desert/hill and desert/floodplain/river tiles).
- The UU is a good turtling UU.
2) Poland
- UA provides over a full SP-tree over the course of a game, including opener and closer.
- UB synergizes well with the UU, and provides extra gold
- UU is very powerful UU, though it does upgrade to Anti-Tank Gun (a bummer)
3)Zulu
- UB gives ridiculous promos to melee units
- UU is a good UU, but, more importantly, allows you to essentially upgrade spear-pike-rifle rather than spear-pike-lancer
- The UA helps with maintenance in the early game, while it helps with XP boosts for the entire game
These are my 3 favorites. I've already played at least ~150 turns as every new civ, and the 3 above I've played a full-game with and won.
Honorable mention would be Indonesia (UA and UB are both quite useful, while UU is hit-or-miss for obvious reasons). I play a lot of Pangaea, since the AI has always been bad at naval combat and amphibious invasions, though BNW seems to have improved the naval side a bit, so, long story short, Indonesia may simply not mesh well with my preferred map-type and is probably one of the best civs in the game for archipelago and islands maps.