Soooooo many choices...and all areas are diverse (think hearing 10 different languages every 3 blocks). I'm sure you'll love it!
It really depends upon your very specific likes/dislikes as this type of thing is what Toronto does best. Pretty much anywhere in the core from the more upscale Yorkville/Annex area to the shops/stalls of St. Lawrence Market, the more entertainment centred King/Spadina, Central King West, West King West, and West Queen West with tons of shops/restaurants/bars/people//parks, Liberty Village is young/hip/cool/professional, Little Italy is fun but can get crazy on weekends, Bloor West Village is always populated but more with families than singles, Kensington Market is similar to Commercial Drive in Vancouver with grit, art, and a vehemently local populace.
I'd avoid the waterfront because while there are people, you always have to go somewhere else to get to decent restaurants and bars - in 10 years this'll be different.
If possible, rent for a month or so and go visit these areas before committing to a year long lease (or purchase). You really need to come and do some exploring as this type of thing is so personal. Rest assured you'll find what you're looking for.