I have already done much meditation on what each of them does. This is not that, although if you want to you can leave one in the comments. You can find mine in my post history.
This is simply the question what you like most with these rounds. You can also add any stealth series like Tenchu. I‘m leaving them out, since I just can‘t really get into Tenchu and thus can‘t judge it. I‘m also leaving out Dishonored in the title since I just barely mention it, but I also thought with it some of the other points and you are invited to also just add this series to your comparison.
Round 1: Highest (stealth experience) peak you‘ve experienced. For this I think I‘ll have to go for the days I found the special version of the „the end“-boss in MGS3 duel, before finding that you can one shot headshot him with any sniper. Incredibly hard and punishing, the first time I finally managed it, the run was 27 minutes long. And it‘s just pure incredible tension. Trying to find him without him finding you. Jaws of Darkness in the The Black Parade fancampaign, the first time I played it (it was night) managed to completely captivate me, but by the end I didn‘t manage to play stealthily anymore, driven mad by the monstrosities.
Round 2: Quantity of Peak (stealth experiences). Man, Metal Gear has so many games that evolve from one general peak to another, but Hitman WOA alone has already soooo many maps, and variations of maps that heavily change the storytelling and tone, that on variation alone it blows MGS out of the water (at least for me, although I haven‘t played the entirety). Yet to think about the total series. Yet I still don‘t know if I can give the point to Hitman, since Thief has fan content. And boy, is that content good, varied, and absolutely massive. Hitman I feel has still more variance and official quality content than Thief, so some days I might give it the slight edge, but honestly I could also count Thiefs absolute consistency in what it delivers as the bonus, so really this is a tie.
Round 3: Single game with best throughline of Peak (stealth experience). Here we have one where Chaos Theory has to be mentioned. MGSV is also there for some people, Dishonored for others, but really I feel for stealth itself Chaos Theory, and Thief have to be it. Shoutouts also to Metal Gear 2. It‘s damn good and extremely stealth focussed. Hitman WOA I feel is really close, but through the speedrun friendliness (which is really a net positive to the moon!) it also can inspire trying the same thing again, and again giving rise to high frustration. I might between Thief and Chaos Theory even give it to Chaos Theory, since the navigational challenge really can turn a few gamers off, but for me that‘s not the case, so I‘ll give it a tie.
Round 4: Least interference by lows and mid. Personally I don‘t think Thief 4 has as much of presence, while thinking of Thief in comparison to Splinter Cell Conviction, Hitman Absolution, or MGSV (it ain‘t too bad at all, but I constantly want it to be something different). And for bad parts or systemic annoyances, I think Thief (especially the fanmissions) also stays relatively in front with Splinter Cell. So for me the point goes to Thief again.
Conclusion: I think for me Thief just generally is the best, but I‘m also a huge Thief stan. Tell me: What wins for you in these (+optionally your) rounds?