Which is the best Dad quote in the MCU among all of them, according to you?

That’s the one, thank you!

No. Cause If you’re describing a timeline in which any variant of tony traveled to the past to meet his father, by definition that has to be a branch timeline and not the prime timeline, as any visiting of the past from any timeline creates a new branch timeline starting from the moment of the visit. The prime timeline Howard will never have interacted with his future son by the rules of MCU time travel and quantum multiverse theory no matter how many times Tony visits the past.

And what you’re suggesting would require targeted multiverse travel, not just time travel (Which apart from America Chavez is impossible, and she still cant go into the past without creating a branch timeline). A variant can’t travel into a past that hasn’t happened on their timeline, but when they do travel to their past it immediately creates an offshoot from the timeline that they traveled from at the point in history where they travel to. So it is not possible for the prime Howard to ever receive this visit or this quote from Tony

And to go deeper, until Sylvie killed HWR, there were no variants, as the TVA pruned every offshoot of the sacred timeline to prevent branches from forming. Which when they did, those branches, forming throughout all of time at once became the origin of the present multiverse and the infinite dynasty of Kangs HWR knew would come. So don’t think of the MCU multiverse as one where the Big Bang created infinite parallel universes from the beginning, but rather as one universe, where different potential choices people could make each create an offshoot branch and all of those possibilities exist simultaneously. Like Shrodinger’s cat but on the scale of the universe

There is a variation of the MCU time travel and multiverse rules, one that does not contradict the existing rules and in my opinion fits better with what we see in Loki, where time travel producing small changes can result in a local and temporary branch that essentially remerges with the original timeline. Those local changes would constitute a new parallel timeline only for a brief period of time and space where differences continue to exist until they peter out. This would be extremely common with regards to quantum fluctuations without time travel where, e.g., you choose chocolate ice cream instead of vanilla. A few days later you cannot even remember that you had ice cream that day let alone the flavor, and there remain no further consequences.

Let’s posit a case where Howard originally came up with the quote on his own. Tony’s past would be unaffected by his time traveling and thus while Howard’s experience would be changed slightly the changes would not result in a full new branch to the timeline.

Explicit bootstrap paradoxes are more complicated and I’m not a fan since I can’t think of a way they really make sense, but we do see one explicitly in Ms Marvel. A possible fuzzy explanation would be that magic can be used to forge a bootstrap paradox/stable loop timeline that also is consistent with MCU rules. But I don’t see an obvious way that could work without magic to kickstart the loop since, as you say, traveling back in time to substantially make a change to produce a time loop would result in a separate timeline and then when the next iteration of the time traveler travels back in time they either will create yet another fresh branch from the original or a new branch of the initial branch where the new iteration time traveler runs into the prior/initial iteration of the time traveler. Maybe there’s a way to make a bootstrap paradox some way but I can’t think of one.

I could yet imagine a stable loop that does not causally affect the time traveler though, like Steve always marrying Peggy and staying in the same timeline when he time travels. It’s a stretch but in storytelling terms I think it works well enough.