Technically? As with all things Trek, we have some inconsistency from the fact that the writers write things to be as good as they need to be for the story to work out as planned. In some cases, you see Defiant class thrashing Galor-class vessels and being said to be equivalent of three Keldon classes. In other cases, like the USS Valiant, we see one Galor-class ship just about draw with the defiant class, only for the Defiant-class to be repaired quicker. I would suppose how you would measure it. Straight up firepower, durability, and ability to act as a strategic flagship (or catch/escape in retreat at warp) I would lay money on a Galaxy-class. However, If we take into account that a Galaxy-class simply WON'T take on a Defiant to the end (not wanting to lose the civilians live aboard, more expensive to repair, more valuable as a strategic asset, etc) then in a more realistic setting, the Galaxy would simply retreat once it was clear that battle would be costly and it would safely outpace the Defiant.
Additionally, my brother and I did a test before I posted. We played ST Legacy, him three matches as a Galaxy and me a Defiant, Where I won 2-1 matches, but all were very close matches. Then, to account for player (crew) ability, we switched, and that was 3-0, all in favour of the Galaxy. Those matches were not as close. Not sure how reliable you all consider that test, but it is there for what it is worth.