Mutoh has won a lot of titles but even if you counted every borderline major singles title he's won it doesn't look like he's won 16.
Mutoh has 10 being very generous.
Triple Crown 3x, IWGP Heavy 4X,NWA Heavy 1x (borderline but why not), GHC Heavy 1x, Wrestle-1 Championship 1x (heh)
Tanahashi 8: IWGP Heavy 8 times
Misawa 8: Triple Crown 5x, GHC 3x
Fujinami has at LEAST 7: 6 IWGP Heavys, NWA World, and there's a bunch of others which if I gave to Mutoh I'd probably give to Fujinami
Kensuke Sasaki another guy who did the Triple Crown/IWGP/GHC run has 7: Triple Crown 1x, IWGP Heavy 5x, GHC Heavy 1x
Suwama (who is still active and the current champion) has 7: AJPW Triple Crown 7x which is the record)
Toshiaki Kawada has 6: Triple Crown 5x,, Z1 Heavy 1x
If you start counting every heavy belt or also tournaments it gets even messier. Jumbo won all the elements of the Triple Crown several times before they were united (and 3 times after) but I didn't count those. Chono "only" won the IWGP title once but he won the G1 5 times.
At the end I'd still say Ric Flair continues to stand alone though. WOOOO~