User blog comment:Ubersketch/What is the most powerful OCF?/@comment-35470197-20181104010048

Taranovsky's C is not an OCF. If you meant an ordinal notation system, then answers by many other googologists to my similar question here might be helpful.

Also, there are many OCFs which might be greatly strong but do not generate large numbers by the unformalisability or the lack of associated ordinal notation systems (e.g. UNOCF, OCFs by Hyp cos, and so on). Therefore the answer depends on what "powerful" means for you.