I am leaning towards Nautilus and Shen a bit.

You know what, I am going to buy Nautilus because he is awesome, his abilities are awesome, and he is fast enough for me, which is pretty slow, but whatever. He has that shield, ult, and drag that i am looking for in an ap tank/fighter.

Nautilus is fun, but he doesn't do as much damage as I thought he would be doing

I actually bought nautilus yesterday, and he is really awesome, but the team is complaining how I play. I requested to play mid instead of jungling, and they were surprised when I drew first blood on jayce

pretty awesome nautilus, but there is someone that i have been considering for a while. Is Hecarim a good fit for me or no?

nau or cho would be my choice for the initiation or cc. amumu, sniged, malp, and shen are top banned tanky champs in rank, so your option is little if you play draft/rank.

Of those listed, Nautilus (no speed buff, but Hook can add mobility as well as good for chasing directly, can jungle, can support, has solid magic damage for the "tank" role) or Volibear (speed buff and slow great for chasing, can jungle, can top, is more bruiser and AD focused) feel like the most fit your criteria.

Cho isn't good for chasing, but does have good flexibility and functions well as an AP tank. Amumu is a little better at chasing than Cho by comparison, but doesn't have as much Flexibility. Sejuani is in a similar state to Amumu, though she's a bit better at chasing than him, but she also doesn't really function as an "AP Tank" rather than just pure tank.

Singed as already mentioned is a great choice if you don't already have him. Rammus, though slightly less AP focused and less flexible, but is a fantastic tank for chasing enemies, and is very capable of pushing if/when he's in a lane (both minions and towers).