I mean yeah, he isn't maybe the strongest. He is, at least stronger than any other we ever seen, little known fact, while it was cut from the anime, the manga has several key quotes that say Dio's power multiplied from when he initially turned to his casual stremgth by the end, not only that but he's stated to be several times stronger than Will and Dio had an off screen training montage on top of all the women he ate boosting his power (not made obvious in the anime but in the manga, the more flesh a vamp consumes the more power boosts one gets.) Basically, it's made evident that end of Part 1 Dio is multiple times stronger than Straizo, who should be around the strength of Dio when he furst turned, given Straizo himself admits he hasnt bothered testing his limits and he hasnt ate women and trained fo two weeks, on top of being fresh.
Honestly, all things considered HIGH DIO should be stronger than Santana given the difference between Santana and Straizo was only like 3-4x and Dio is several times stronger than Straizo, seems a bit contradictory but Dio is hot shit among his type. Of course Wham is a **** ton stronger than Santana but HIGH Dio should be close to Wham's strength, but being close doesnt mean you can punch holes clean through others.
It's odd but going by the multipliers given and random bits of strength info we get on characters, High Dio should be around Pillar Men strength, not quite Wham of course and definitely not quite Kars but he should be above Santana, which kinda ***** with the Pillar Men>Vamps scaling, but that was Dio with a huge power boost and Santana is below average for a Pillar Men so eh.