1. Spain sucks, every time I have played them I usually find one wonder at the other side of the map (I play standard fractal).
Spain is 9 times out of 10 like playing a civ with no UA. Then that one time you have a good start (I am still waiting) I'm sure it's like hitting the lottery (a gimick). I havent had one good start with them ever. I seriously dislike them.
Now I am sure there are players that play spain and every time you get killer wonders right at your doorstep, but I can only go by my experience. I NEVER EVER get anything with them (like playing a civ with no ua).
2. Never bothered to play America (seems weak).
3. Ottomans have a sucky UA they are a joke. (unless playing island map which I dont ever play)
4. England sucks too for having a crap UA.
5. Russia sucks because who needs all those resources? maybe in MP you would but I do SP. Russia could be a mid tier civ for mp but I think they really are low tier for sp games.
Those 5 civs really stand out as being very weak and bad.
I am going to say there are 2 civs that seem really OP. France and Persia for building (I dont have the xp pac with babylon so cant comment on them).
Siam and Greece are good I suppose but I dont like messing with city states all the time. Thats just a playstyle thing for me.
For warmonger I would say Mongal, China, Japan.
When looking at this game there seems to be a few very powerful elite civs, a bunch that are medicore in the middle, and then a few that reallys stink.
Sadly the civs are not well balanced in my opinion. I would hazard a guess that the majority of civ players only play a small percentage of the "popular" civs since many of the lesser civs are a total joke.