I've bought out my other S-Corp members and now I am the only owner. We used to file a 1120 w/ K1s. Since I'm the only one left, do I file stuff uncder Schedule C?

S-Corps file an 1120S regardless of how many shareholders there are.

you still have an S corp that needs to file a 1120-S.  failure to do will probably get you a notice from the IRS,  Then you will pay penalties for each month you failed to file. Not knowing what's in it, I can give you no guidance as to any tax consequences of liquidating.   Sometimes there are adverse tax consequences.  Seek the advice of a tax professional.  this is not a do-it-yourself project.    in the year liquidated you would file a final return.   

Each state has its own rules as to what it takes to liquidate and not leave you with possible future liability besides taxes.   

again pay for a tax pro.     

An S-Corp ***CAN*** have only one owner. So the fact you bought out all the other members and are the only remaining owner of the business does not change the fact that you will file an 1120-S. All members you bought out will be issued a "final" K-1 for them to show they disposed or sold of their share and interest in the business, to you.