Epcot is fastest since it is non-stop from there. To get to DHS from Riviera, as noted above, requires you to change lines at CBR so you have to figure in the transfer time.
If we have a Topolinos breakfast reservation at 8:30am and are staying at Pop Century, what’s our best option to get there? Get out at CBR and walk over? Or can we transfer from CBR to Riveria even though technically the Epcot transport doesn’t open that early.