Generally I prefer Lowe's over Home Depot and Rona.
The staff at Lowe's are always very helpful, unlike HD where they can barely point you to the right aisle to find a hammer.
I find the selection to be a bit more varied compared to Rona and I have a better chance of finding what I need. Not that I can really complain about Rona's selection either, its just more of a preference.
Probably the best part of the shopping experience here for me is the self-checkouts. They actually work well here. I find the one's at HD always have a hiccup which forces a cashier to come over and help. I've only ever had one problem at Lowe's when a tool box I bought only had an item number and not a UPC which their system doesn't allow you to type in manually.