I think, at least within the lore of KH itself, it is impossible to actually "rank" wielders because there are far too many factors to consider and there isn't actually an in-built definitive system for us to compare people with. (It's not like this is DBZ with power level scouter things, after all).
Any assessments on "strength" are going to entirely be subjective as a result. And KH has already introduced so so so many different types of strength. We have strength of heart, will, resistance to the darkness, pure physical strength, magical ability, etc. I mean, without knowing exactly what measurement you are going by it is going to be hard to determine any definitive answer.
I think every single wielder excels at something different better than the others and that is what makes them all unique and it would be impossible to throw them all into a death match and say "ah, yes, for sure Ven will win this!"
Besides, we have no idea yet WHY Ven (and Lauriam to some extent) are the only Dandelions still around (as far as we know), so simply being one, I think, doesn't automatically grandfather him into top dog position. If that were the case, shouldn't you also be arguing that Lauriam is better than Sora, even though he very obviously lost their fight together in Castle Oblivion?