I’d say the way Big Mom specifically talks about Whitebeard puts them above.
Look at how she mentioned the Yonko in WCI when discussing Lokis failed marriage:
“I’d have defeated Shanks, Kaido, and EVEN WHITEBEARD…” Big Mom herself put Whitebeard on a level separate from Shanks and Kaido. Everything we’ve seen and heard from she and Kaido consistently keeps Roger and WB (and Shiki/Garp/Sengoku by extension) as a separate level back in their primes
Look at how she mentioned the Yonko in WCI when discussing Lokis failed marriage:
“I’d have defeated Shanks, Kaido, and EVEN WHITEBEARD…” Big Mom herself put Whitebeard on a level separate from Shanks and Kaido. Everything we’ve seen and heard from she and Kaido consistently keeps Roger and WB (and Shiki/Garp/Sengoku by extension) as a separate level back in their primes
But Shiki was never called Roger equal like WB , Sengoku pretty much stated Shiki would lose to Roger in the end . Shiki was praised as great pirate on pair with WB / Roger but his individual power level was never hyped as those 2. SHiki also admitted after Roger death its WB era now.
Nothing for me to put Shiki above Kaido / Big Mom at least so far.