If you guys are interested only about whats OP, then OP is anything with unusually high win loss ratio, since win/loss ratio is the only factor taking EVERYTHING into consideration. It goes something like this:
46%- an automatic defeat champion, picking this is a proof of your sheer stupidity
47-48% an UP champion but can work in some rare cases
49-51% a balanced champion, in other words a meh champion
52-53% an OP champion, very strong strategic pick
54%+ an incredibly imbalanced free win champion, if this champion isnt banned then it is your duty to flame the banning player
Note that a champion can have drastically different win ratio in unviable game modes (anything other than ranked solo queue and ranked premade 5v5).
But I assume you are just a troll looking for attention, which I have given you. Oops.
Well sorry to burst your bubble but Shen is indeed quite horrible at the moment. He failed miserably at world finals and he fails regularly in most soloq games too as his ult is pretty much the only meaningful thing he can do and that just doesnt cut it. His selling point (split pushing) is irrelevant because during the time he needs to destroy one tower enemy Sivir will destroy 5 towers + nexus.
Rammus has great w/l because he is excellent at taking down towers which is extremely valuable. If his ult didnt affect towers his win ratio would likely drop by about 10%.