Considering the "crimes" both Ampleforth and Parsons committed, which would you think the Party would consider most heinous and why?

The Party really doesn't care about Ampleforth or Parsons: they are simply disposable.

Ampleforth's crime:

‘These things happen,’ he began vaguely. ‘I have been able to recall one instance—a possible instance. It was an indiscretion, undoubtedly. We were producing a definitive edition of the poems of Kipling. I allowed the word ‘God’ to remain at the end of a line. I could not help it!’ he added almost indignantly, raising his face to look at Winston. ‘It was impossible to change the line. The rhyme was ‘rod”. Do you realize that there are only twelve rhymes to ‘rod’ in the entire language? For days I had racked my brains. There WAS no other rhyme.’

Parson's crime:

‘Thoughtcrime!’ said Parsons, almost blubbering. The tone of his voice implied at once a complete admission of his guilt and a sort of incredulous horror that such a word could be applied to himself.

‘Of course I’m guilty!’ cried Parsons with a servile glance at the telescreen. ‘You don’t think the Party would arrest an innocent man, do you?’ His frog-like face grew calmer, and even took on a slightly sanctimonious expression. ‘Thoughtcrime is a dreadful thing, old man,’ he said sententiously. ‘It’s insidious. It can get hold of you without your even knowing it. Do you know how it got hold of me? In my sleep! Yes, that’s a fact. There I was, working away, trying to do my bit—never knew I had any bad stuff in my mind at all. And then I started talking in my sleep. Do you know what they heard me saying?’

He sank his voice, like someone who is obliged for medical reasons to utter an obscenity. ‘’Down with Big Brother!’ Yes, I said that! Said it over and over again, it seems. Between you and me, old man, I’m glad they got me before it went any further. Do you know what I’m going to say to them when I go up before the tribunal? ‘Thank you,’ I’m going to say, ‘thank you for saving me before it was too late.‘‘ ‘Who denounced you?’ said Winston. ‘It was my little daughter,’ said Parsons with a sort of doleful pride. ‘She listened at the keyhole. Heard what I was saying, and nipped off to the patrols the very next day.