Absent other constraints, my preference for the OL route west of Yonge would be straight along Queen West, and then up Dufferin and continue north of Bloor to relief the Dufferin transit.
However, given that OL Phase 1 will join the LSW rail corridor and hit the Exhibition GO, it might be a good idea to continue further west within the same rail corridor. That would be a lot cheaper (at-grade extension) than tunneling up Dufferin, and likely much easier to build. In order to continue up Dufferin, the line would have to go from surface to tunnel at the same time as it curves from westbound to southbound.
If OL continues in the rail corridor, then Dufferin could eventually be served by another subway line. Roughly Eglinton to College, then curving towards downtown and crossing the downtown under Dundas, acting as Relief Line 2.
Edit: it should be noted that the Dufferin bus is extremely busy between Eglinton and Queen. South of Queen and north of Eglinton, that bus is well used but not too crowded. Thus, if an underground line covers all or almost of all of that segment, Eglinton to Queen, it will do the job of fixing the Dufferin transit.