I picked Bogaerts and Betts as my first two, but I think it is a disservice to the Red Sox leverage to call them prospects. Looking at the whole picture, Bogaerts has put up a league average line for a SS over a full season in the majors before the age of 21 and showed enough defensively to stick there. Betts has similarly lost his rookie eligibility, while excelling offensively even as he was learning a new position. No way I'd let a trading partner get away with calling them prospects. These guys have proven they aren't Jackie Bradley.
My other two were Owens and Rodriguez, because I don't think you can afford to give up anyone with even as little as a 10-15% chance of being a top 25 starter in MLB. The cost of acquiring such a rare talent in trade or free agency is ridiculous and so I'd hoard any possible opportunity to get one from the farm.
That put me at the poll limit, but because I don't count Betts and Bogaerts, I'd add Swihart and Devers to my list. These two seem like they have the highest upside you can have in a prospect (as do betts and Bogaerts). In particular, Devers would be ahead of Swihart for me, because of his level would be nowhere near valued rationally in a trade. They'd almost surely be selling low. I'd love to see a study, but from 30 years of following the minors, Players with his pedigree that excel in rookie ball at that age rarely flame out in low A, and a successful Greenville season will add exponentially to his trade value. I'd loathe giving up Swihart, but he likely would be fully valued and I have a fair degree of confidence in Vazquez being well above average as well.
As hinted in that last sentence, I don't actually define untouchable as strictly untouchable. However, to give up even one of these guys, I'd need to be getting back at least 3 years of below market control from a prime aged perennial all star at a position of need with the rest of the organizational depth remaining intact.
In a package of 3 or less players with no other top 10 prospects, I'd give up Owens or Rodriguez for 5 years of Hamels. I might be tempted to give up any one of the non-Bs in a similar package for 3 years of Strasburg, and I would give up even Betts for 3 years of Bryce Harper. But that's what you'd have to be talking, which is as close as you get to untouchable.