If we are talking the entire series, im quite sure Hades is the strongest. Mages like Mavis and Zeref dont count considering their in a class of their own. Now if we're talking active mages and ones we have seen to give an opinion of then Makarov. Gildarts is strong no doubt about it but makarov is much more experienced and has more in his arsenal. Titan magic,Fairy law,light magic,high level defensive spells,and more elemental magic that we might not know about. Gidlarts is reckless and has his hand to hand combat style but at range his magic isnt gonna do anything. I'd say speed,power,and spell wise jura and gildarts are equal. Makarov is way up there but the only problem with makarov is his endurance and health. Jellal is quite strong so i guess i'd put him below jura/gildarts.
Having just defeated Jura of the ten saints, I think Laxus climbed the rank if there was any doubt of him being the strongest in Fairy Tail... with the exception of uber Gildartz.
Inaccurate Power Rankings:
1-4 Ten Wizard Saints Mentioned (Not Human)
7 Laxus, Natsu, Gajeel