When he wasn’t interested in Sir Cadogan’s noble quests

‘Farewell!’ cried the knight, popping his head into a painting of some sinister-looking monks. ‘Farewell, my comrades-in-arms! If ever you have need of noble heart and steely sinew, call upon Sir Cadogan!’
‘Yeah, we’ll call you,’ muttered Ron, as the knight disappeared, ‘if we ever need someone mental.’
Harry Potter and the Prisoner of Azkaban

When he helpfully and calmly reminded Hermione of a key skill of hers

‘Devil’s Snare, Devil’s Snare ... What did Professor Sprout say? It likes the dark and the damp –’
‘So light a fire!’ Harry choked.
‘Yes – of course – but there’s no wood!’ Hermione cried, wringing her hands.
‘HAVE YOU GONE MAD?’ Ron bellowed. ‘ARE YOU A WITCH OR NOT?’
Harry Potter and the Philosopher’s Stone

When he devised an expertly off-kilter method for destroying Lord Voldemort

‘What are you doing with all those books, anyway?’ Ron asked, limping back to his bed.
‘Just trying to decide which ones to take with us,’ said Hermione. ‘When we’re looking for the Horcruxes.’
‘Oh, of course,’ said Ron, clapping a hand to his forehead. ‘I forgot we’ll be hunting down Voldemort in a mobile library.’
Harry Potter and the Deathly Hallows

When he was really supportive of Harry during the Triwizard Tournament

Harry’s stomach leapt – he was now tying for first place with Cedric. Ron and Hermione, caught by surprise, stared at Harry, then laughed and started applauding hard with the rest of the crowd.
‘There you go, Harry!’ Ron shouted over the noise. ‘You weren’t being thick after all – you were showing moral fibre!’
Harry Potter and the Goblet of Fire

When Ron gave us all a really valuable Defence Against the Dark Arts lesson

Harry took a deep breath and continued calmly, though his insides were boiling, ‘Yeah, ghosts are transparent, but Inferi are dead bodies, aren’t they? So they’d be solid –’
‘A five-year-old could have told us as much,’ sneered Snape. ‘The Inferius is a corpse that has been reanimated by a Dark wizard’s spells. It is not alive, it is merely used like a puppet to do the wizard’s bidding. A ghost, as I trust that you are all aware by now, is the imprint of a departed soul left upon the earth... and of course, as Potter so wisely tells us, transparent.’
‘Well, what Harry said is the most useful if we’re trying to tell them apart!’ said Ron. ‘When we come face to face with one down a dark alley we’re going to be having a shufti to see if it’s solid, aren’t we, we’re not going to be asking, “Excuse me, are you the imprint of a departed soul?”’
Harry Potter and the Half-Blood Prince

When he simply wanted to look at Uranus

Seamus and Dean, who were working nearby, sniggered loudly, though not loudly enough to mask the excited squeals from Lavender Brown – ‘Oh, Professor, look! I think I’ve got an unaspected planet! Oooh, which one’s that, Professor?’
‘It is Uranus, my dear,’ said Professor Trelawney, peering down at the chart.
‘Can I have a look at Uranus, too, Lavender?’ said Ron.
Harry Potter and the Goblet of Fire

When he tried to out-Hermione Hermione

‘No,’ said Hermione shortly. ‘Has either of you seen my copy of Numerology and Grammatica_?’
‘Oh, yeah, I borrowed it for a bit of bedtime reading,’ said Ron, but very quietly.
Harry Potter and the Prisoner of Azkaban

When he was the grandmaster of tact

The three of them fell silent. After a long pause, Hermione voiced the knottiest question of all in a hesitant voice: ‘Do you think we should go and ask Hagrid about it all?’
‘That’d be a cheerful visit,’ said Ron. ‘Hello, Hagrid, tell us, have you been setting anything mad and hairy loose in the castle lately?’
Harry Potter and the Chamber of Secrets

When Draco Malfoy became a ferret

‘Don’t talk to me,’ Ron said quietly to Harry and Hermione, as they sat down at the Gryffindor table a few minutes later, surrounded by excited talk on all sides about what had just happened.
‘Why not?’ said Hermione in surprise.
‘Because I want to fix that in my memory for ever,’ said Ron, his eyes closed and an uplifted expression on his face. ‘Draco Malfoy, the amazing bouncing ferret...’
Harry Potter and the Goblet of Fire

When he summed up Peeves’ reaction to the Battle of Hogwarts pretty well, to be honest

Somewhere in the distance they could hear Peeves zooming through the corridors singing a victory song of his own composition:

We did it, we bashed them, wee Potter’s the One,
And Voldy’s gone mouldy, so now let’s have fun!

‘Really gives a feeling for the scope and tragedy of the thing, doesn’t it?’ said Ron, pushing open a door to let Harry and Hermione through.
Harry Potter and the Deathly Hallows