Not too much into the debate, but I think everyone is underplaying the GOI. all because the author decided to focus only on FT members.
While making strategies, i would assume that Mavis made it in consultation with Makarov along with her own intelligence. She is said to be master strategist, so when i am basing on manga facts, i think I will have to take her ranking on fighters as correct.
Look at
this page where Mavis states that The GOI is their strongest force which is needed to eliminate the greatest threat from East. She also knows Warrod personally, so she must have known that he is strong enough to be included in the Strongest force of the continent.
It is not something new that Shonen authors often introduce many characters, hype some of them to epic length and later can't deal with them. Mashima thinks that whole Alvarez empire must be defeated by Fairy Tail members, so he is gonna write according to that. This is not right, there should be some aged and experienced strong fighters. but no, all we have are FT members and their friends who are all part of younger generation (except 4 or 5). as if a generation of Mages forgot to train and thus no one became strong enough. It is the major problem for most of shonen manga, as if younger generation is the best of them all. It is applicable for Naruo and Bleach completely, One Piece is better in this regard.
Now on topic, GOI was said to be strongest by Mavis and the claim was supported by other mages; Jura wasn't originally part of GOI, so I am going to consider that. When God Serena defeats them off panel, it means that God Serena is lot stronger than the GOI. The only reason he was downplayed by Spriggans because he was newly added member.