Since I don't remember much before 2004-2005 since I was only around 10-11 years old, here are my top 5 since then;
1) February 7-8, 2013: This storm dropped 10-18" across the GTA and is one of my personal favourites because up until this storm, that winter was pretty mediocre. 9 hours of straight heavy snowfall and zero visibility with temperatures around 17F made it an amazing storm.
2) December 15-16, 2007: This storm dropped around 10-14" across the GTA and was the start of what later became one of Toronto's snowiest winters on record (as for many in this sub-forum). Heavy snowfall combined with near zero visibility for hours makes this one of my top 5 storms.
3) February 6-7, 2008: This storm dropped 12-16" across the GTA and was in between two other storms that dropped 6-9", respectively, in a span of 12 days making it Toronto's snowiest February on record. What makes this storm great is the Thunder snow and near zero visibility through-out the evening and overnight.
4) December 21-22, 2013: Though this is technically not a snow storm, it is a winter storm because it became known as one of Toronto's worst Ice storms. Total ice accretion was over 1.2", causing significant damage while solidifying the existing snow base which didn't melt till late March.
5) February 4-5, 2014: This storm dropped around 8-12" across the GTA and was the storm that really strengthened our snow depth to over 20", and helped us have a new record for the number of consecutive snow cover days (~104-110 days).