For me, it's definately the most recent one. I forget which opening numeral it is, but the song is "Change". I also love one of the earlier ones; again not sure of the numeral. The song there however is "Ichirin no Hana" by High and Mighty Color (I believe)... didn't take me long to fall in love with it haha.
I personally find ranbu no melody and Sen No Yoru Wo Koete are two of the best, I think I may like another just as much, but I love them all, and I the girls who sang "Change" Also made a cover of The aqua timez opening called Alonez.