I'd also say Marco seeing as Marco was a peak member of a Yonkou crew whereas Sabo is still young and has more growth left so Oda will probably still have him below the likes of Marco and other top Yonkou members. Additionally Marco I'm quite sure was implied or stated to be>>Ace and as kids Sabo and Ace were of similar strengths (while Luffy was a lot weaker, albeit younger) so perhaps Oda will have left it so that Sabo is at the level Ace would be at if he was youknow still after the timeskip.
Not actually seen Sabo yet apart from just a few glimpses in the anime.