First off let's differentiate between tank and initiator. A tank is somebody who can babysit really well, be it taunts/stuns/knockup/etc on a decent cooldown. Saying amumu is a tank once every 3 minutes or so is ludicrous. Just call him an initiator and call it a day.
Rammus = nearly perma ban at this point, but is one of the best.
Malphite is less of a tank and more of an initiator but very strong as well.
Amumu is like Malphite less of a tank and more of an initiator but also very strong. Arguebly has the most aoe dmg out of all on this list.
Shen is not as strong of a tank like Rammus but provides a degree of utility that can't be matched. 2v2 becoming a 3v2? Yes please.
Alistar is decent as well, with good formation disruption or babysitting abilities, He also has the best ap ratios around 1.0 on all his dmg ability which makes ap alistar the lulz.
Galio is decent as well, with the best poke out of all tanky ppl on this list, but his ultimate is hard countered by merc treads + flash.
You could go on with nasus, renekton, etc etc. Most champs can be built tanky and be called a tank, but the above is just a list of the commonly known ones/builds.