Interesting idea this one. I would take Titan Trypticon.
It has a cool base mode, nice decent looking spacehsip mode, IS HUUUUUUUGE and great dinosaur articulation. Considering that mid20s year old me had an awesome time playing with my RID Fort Max as a base, I would say 12 year old me would have an absolute blast with his options and play factor.
Titan Ark would blow my mind as well too, mainly because I'd be all like "Hell yeah! they made an Ark."
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I don't have it it, but the Haslab Unicron would've been like nectar from the heavens. And also as frustrating as hell I suspect.
Although to be completely honest, just seeing a toy with the amount of articulation and character they have now would've blown my mind. Like comparing G1 Optimus with Earthrise for example. Boom! Ankle tilts are standard!!