Really the most memorable 20th Century PMs are Churchill, Atlee, and Thatcher (and maybe Blair, maybe).
The claims that the sinking of the Belgrano was a war crime always makes me laugh, an armed battle ship sunk in the middle of an armed conflict, and the argument was, what, she was sailing away from the islands? Seriously do people have any concept how quickly she could turn around? After Belgrano the Argentinian navy basically returned to port, and that likely saved British lives. Never start a war then complain when the other side come along and smacks you in the face.
Northern Ireland, I imagine having lost friends and collegues to the IRA probably clouded her judgement somewhat, but again public support in Britain was behind her on this, and you could argue that beligerence is one of the factors that eventually convinced the IRA et al (for the most part) that this wasn't a conflict that could be won through force of arms.
Destroying the mining industry? It was already well in decline but why let facts get in the way of dogma
As for British industry, again things aren't so clear cut as people would have you believe
and no one ever talks about how much a factor in the destruction of British industry the intractableness of the Unions often was.
If only Scargill had actually baolloted his men eh?
In reality the only way to truly know what impact (positive or negative) Thatcher had would be to visit an alternate universe where she never won the '79 election, and since that's not going to happen people will just choose their sides, choose their facts. I think History will look at her objectively highlighting the good and the bad. She wasn't a saint but she wasn't a monster either.