If it's about strength, then:
1. Jellal and Erza
2. Gray and Juvia
3. Natsu and Lucy
4. Gajeel and Levy
But Nalu is still my OTP <3

1.Gray x Juvia
2.Natsu x Lucy
3.Jellal x Erza
4.Gajeel x Levy

Really Erza and Jellal are CRAZY strong!!! and on top of it they are working together!!! a S-class monster and member that used to be in the magic council. Jellal!!

natsu and lucy hands the girl is stronger then anyone gives her credit for and natsu is more powerful then any guild member outside of makarov and geldarts

Natsu and erza?

i think it is erza and jellal, she is an s-class mage and he is a member of the magic council at 19 and plus he is a former member on the ten wizard saint
the second will be gray and juvia, gray is pretty strong and juvia is a former s-class back in phantom lord
the third will be natsu and lucy, natsu is strong but lucy is not, even i think she can't do anything without her spirits
the last will be gajeel and levy, gajeel is not as strong as natsu

For each of them, I think they would win all the way. Nothing beats companionship and friendship. :)