+1 to Russell Hill Road (grab a copy of the city cycling map, the route Cavit8 mentions is basically the southbound city route from up that way - I've used that and variations of it get up to Yonge/Sheppard and Yonge/Eg a bunch) - it's a great way down, sweet descent. I also like Dupont for traversing the city in that area. I've not much experience specifically with Leslie or York Mills or that particular area.

I came down Dufferin a bit tonight (from Eglinton to St. Clair anyway) - man that stretch is wild and not particularly smooth. Went across St. Clair to Lansdowne and south all the way to just under the bridge south of Dundas - the hill coming down toward Davenport is good practice for braking sans-brakes (and a bit of skiddy skid fun )

edit: I ran 42/17 last winter (with platforms, aieeee!) and it was really nice - I think 46/17 as I have now will be fine, but I'm going to hope I can grab a cheap beater and convert it and run a lower gear depending on how the winter seems to take shape.