She certainly knew how to put someone in their place – no matter how good-looking or golden-haired they were.

After Gilderoy Lockhart repeatedly boasted of knowing where the Chamber of Secrets was, and how to defeat the monster that was hiding inside, Professor McGonagall finally had enough.

‘We’ll leave it to you, then, Gilderoy,’ said Professor McGonagall. ‘Tonight will be an excellent time to do it. We’ll make sure everyone’s out of your way. You’ll be able to tackle the monster all by yourself. A free rein at last.’
Lockhart gazed desperately around him, but nobody came to the rescue.
Harry Potter and the Chamber of Secrets

Her tongue was as sharp as her mind

Ouch, Harry – need some ice for that burn?

‘Why didn’t you send us a letter by owl? I believe you have an owl?’ Professor McGonagall said coldly to Harry.
Harry gaped at her. Now she said it, that seemed the obvious thing to have done.
‘I – I didn’t think –’
‘That,’ said Professor McGonagall, ‘is obvious.’
Harry Potter and the Chamber of Secrets

Her delivery was so dry that even the Aguamenti spell couldn’t save you

It seemed that Professor McGonagall didn’t have much faith in Divination as a magical practice, and she had even less faith in Professor Trelawney’s yearly predictions of death at the school.

‘Please, Professor, we’ve just had our first Divination class, and we were reading the tea leaves, and –’
‘Ah, of course,’ said Professor McGonagall, suddenly frowning. ‘There is no need to say any more, Miss Granger. Tell me, which of you will be dying this year?’
Harry Potter and the Prisoner of Azkaban

She never minced her words

When it came to Professor Umbridge, McGonagall’s contempt reached peak levels.

‘I should have made my meaning plainer,’ said Professor McGonagall, turning at last to look at Umbridge directly in the eyes. ‘He has achieved high marks in all Defence Against the Dark Arts tests set by a competent teacher.’
Harry Potter and the Order of the Phoenix

And she simply could not be intimidated

You think holding a position of authority over Professor McGonagall would protect you from her withering glare? Think again, Professor Umbridge.

‘Now, the Vanishing Spell –’
‘Hem, hem.’
‘I wonder,’ said Professor McGonagall in cold fury, turning on Professor Umbridge, ‘how you expect to gain an idea of my usual teaching methods if you continue to interrupt me? You see, I do not generally permit people to talk when I am talking.’
Harry Potter and the Order of the Phoenix

She knew when to stand her ground

And you really don’t want to be the one that she’s fighting. Unless you fancy singlehandedly tackling a stampede of desks.

‘Get back!’ shouted Ron, and he, Harry and Hermione flattened themselves against a door as a herd of galloping desks thundered past, shepherded by a sprinting Professor McGonagall. She appeared not to notice them: her hair had come down and there was a gash on her cheek. As she turned the corner, they heard her scream: ‘CHARGE!’
Harry Potter and the Deathly Hallows

She didn’t approve of foul language, so be sure to watch your mouth

Unless it was absolutely necessary, and then she can curse with the best of them.

‘YOU CHEATING SCUM!’ Lee Jordan was howling into the megaphone, dancing out of Professor McGonagall’s reach. ‘YOU FILTHY, CHEATING B–’ 
Professor McGonagall didn’t even bother to tell him off. She was actually shaking her fist in Malfoy’s direction; her hat had fallen off, and she, too, was shouting furiously.
Harry Potter and the Prisoner of Azkaban

In a battle of the minds, Professor McGonagall could tear your argument to shreds.

If she’d attended a Muggle school, she would have dominated the debating team. You just know it.

‘And what is your evidence for that?’ cut in Professor McGonagall.
‘I have testimony from Willy Widdershins, Minerva, who happened to be in the bar at the time. He was heavily bandaged, it is true, but his hearing was quite unimpaired,’ said Umbridge smugly. ‘He heard every word Potter said and hastened straight to the school to report to me –’
‘Oh, so that’s why he wasn't prosecuted for setting up all those regurgitating toilets!’ said Professor McGonagall, raising her eyebrows. ‘What an interesting insight into our justice system!’
Harry Potter and the Order of the Phoenix

Lastly, here’s one very good reason to keep Professor McGonagall on your side. She was loyal to the last and always defended her students.

‘Humph,’ snorted Professor McGonagall. ‘It’s high time your grandmother learned to be proud of the grandson she’s got, rather than the one she thinks she ought to have – particularly after what happened at the Ministry.’
Harry Potter and the Half-Blood Prince

We love you Professor McGonagall. Please don’t give us detention.