UK, of course.
Followed by Spain, Poland and Canada, I guess.
Overall, 2015 sucks election-wise though.
You got 3 of the 6 biggest countries in EU: UK, Spain, Poland + crisis ridden Portugal and Greece.
Mexico and Argentina in Latin America. Canada in North America.
Nigeria, Ethiopia and Tanzania in Africa.
In Asia you got Burma as a very exciting one + Turkey. Probably also Thailand.
Mabye I'm a bit picky, but other than the UK there are no truly "big ones" in 2015 (as in no significiant US, French, Russian, Japanese, Indian, German, or at least Brazilian elections).
2016 (America, and domestically five German state elections) and 2017 (France & Germany) seem to be more promising.