According to the new eBird "Target" function, the easiest 20 new Ontario birds for me to find in Ontario, with highest probability first, would be:
Pine Grosbeak, Gray Jay, Hoary Redpoll, Boreal Chickadee, Lapland Longspur, Hooded Warbler, Cerulean Warbler, Red Crossbill, Black-billed Magpie, Le Conte's Sparrow, Thayer's Gull, Brewer's Blackbird, Yellow-breasted Chat, Sharp-tailed Grouse, Spruce Grouse, Barrow's Goldeneye, Summer Tanager, Connecticut Warbler, Northern Hawk Owl and Arctic Tern.
As you can tell I haven't done much birding up north.