I would separate Tony's ruthless moments by violent acts vs. non-violent acts.

Non-violent ruthless moments (not in particular order):
1. Giving Meadow Erik Scatino's car and then berating her when she wouldn't accept it. "And the money that pays the bills, buys the cds, and all your other shit" (or something to that effect)

2. Bashing Big Pussy's wife's car after she tried to squeeze money out of Tony for her dog's operation. Tony made it abundantly clear that he wouldn't be shaken down and that he would gladly kill her.

3. Tainting as many lawyers as he could so Carmella couldn't hire them

4. Hooking up with Christopher's goomah (the one who was an ex-druggie)

Violent ruthless moments:
1. When Tony bashed the guy's face from Phil's crew who made the pass at Meadow. Later some of the guy's teeth were caught in Tony's pantleg

2. Setting up Adrianna even though he didn't carry out the hit
3. Killing Ralph
4. Killing the "rat" when taking Meadow on her college visits and then lying to Meadow's face only a few minutes later.

I disagree about killing Christopher. Tony always regretted promoting Christopher and I think he should have been killed earlier in the series. At the very latest, Tony should have killed him when Christopher came into the Bing with a gun thinking that Tony had fooled around with Adrianna. The way Christopher ended up dying was a terrible scene, in my opinion.