I voted for Gekiranger. To me, it felt like the most powerful out of them.
Go-onger's OP is a fun little song.
I like Shinkenger's OP. Though to me, it got a bit boring at times. XD
I might've voted for Goseiger's instead, since I really love the OP. Though Goseiger hasn't finished airing yet. And I personally feel like my opinion towards a theme song would depend on how the whole season worked out.