Is Ostiense the best? I've looked around and for some reason, can't find the right exit. Thanks!

The Coliseum has it's own exit called colosio ( not sure about spelling) The train system in Rome has only two lines which I think are the red and yellow lines( been several years so not sure ) You may have to change trains depanding on where you start. Anyway, the coliseum is right outside of the exit.

Don't you mean which station?

Ostiense i believe would be the closest TRAIN station - not metro station.

As explained on your original thread, take the train to Ostiense and then the metro from Piramide to Colosseo.