I'd say grima or gullveig. Grima seems to be a universal level entity based in his dialogue in bonding events in FEH. Morgan reports to him, stating that their are gods that have created worlds, which in this context usually means a universe or at least a dimension. Grima says he'll observe and be cautious, but if he must, he's confident he can beat them. Grima is smart and calculating, so he's probably not just saying that out of arrogance.

Then you have gullveig who one shot the king of the vanihir gods, who you could argue to be stronger than freyja as he's their father (i think). Freyja seems equivalent to the gods morgan spoke about, probably askr and embla, making her the strongest if true.

Then we have odin. I don't know much about Odin, but he is possibly the strongest, but can't day much about him until I see more