I was trying to think of the most ruthless things Tony has done over the 4 seasons, and the top 3 I've come up with are:

1) Almost killing his mother in Season 1.
2) Torturing and killing Matthew Bevilaqua in
Season 2.
3) Killing Ralph over the horse in Season 4.

I also considered him killing his best friend Pussy, but he wasn't really ruthless, and it tormented him.