The fact that she is called the greatest would imply that she's above the other 4 of the 5 strongest mages in the guild which includes Orga (Minerva, Orga, Rufus, Sting & Rogue are the 5 strongest according to the manga).
Also the only time Orga was called the strongest was when she left the guild, funny enough. Furthermore, fan translations of the same chapter has Rufus call her 'strongest' which further supports that it can be translated and interpreted as strongest/most powerful/most formidable/greatest in this sense.
IIRC 'Dai' (大) in Japanese means 'great' while 'saikyo' ('Sai' + 'kyo') (最強) means 'strongest', however in speaking 'dai' can refer to strength given the context of the situation and since Rufus went on to boast that she's stronger than the twin dragons and was speaking about Erza/Kaguya's strength, it would actually support that he was talking about her strength even if he did say 'greatest' instead of 'strongest'.
I don't know Japanese so I can't clarify if this is a situation like that, though I do have the raws. If someone can clarify what's the word and/or if it does refer to power if it is the word greatest, that would be great!
Again idk Japanese but it seems to me that 最強 (Saikyo) is used in the raw which means 'strongest' as opposed to 大 (Dai) which means 'great'. So it seems that Rufus is literally saying that she's the strongest mage in Sabertooth, hence the use of 最強 (Saikyo) instead of 大 (Dai).
With that being said, it's a kid series, for the most part, so it's not likely that Mashima is nitpicking and differentiating between Minerva being the greatest and Orga being the strongest. And this is me ignoring him getting low diffed by Jura while Minerva had a better showing against Erza/Kagura, even though they are weaker than Jura/Laxus.
No comment because I'm not really part of this discussion.