why did regina smile her signature smile at the end of "the tower?"

It seemed مزید to me like a "That sly dog" kind of smile. Like she wasn't that surprised that he was still alive and she does probably think that his survival will make things interesting.

مزید like "Rumple آپ sly dog" type thing I think she has some affection for him and plus she thinks of he's alive maybe he'll help them since he's so powerful.

"Regardless of how many times we've seen Rumplestiltskin and the Evil Queen try to kill each other, there's an underlying love and connection between them," Parrilla says, likening the relationship to that between Regina and Snow White (Ginnifer Goodwin). "She can't hate Snow and she can't hate Rumple. They've all gotten so close, it's a dysfunctional family. No one really wanted him to die and now we see that he didn't. Learning that he's alive is a good thing all around because we need him on so many levels."

I think she's just glad he's alive.

She's happy he's alive. Like Lana کہا there's "a love and connection" between them. And they're "partners in crime"! xD Also, I think it was like she was amused سے طرف کی how he always manages to survive and had been expecting it for a long time. And Regina knows he'll somehow be useful and was the first one to get it. Plus, they're all family now haha. I think she missed him, I fangirled- a lot <3