My favs are based on total enjoyment experienced, not scale fidelity.
I had to think long and hard (a sure indicator that the question is a good one!), and it has to be Monogram’s 1/48 SBD Dauntless at the top. It had it all - a cool WW II warplane with moveable parts that actually worked well (which went a long way to facilitate precision dive bombing attacks on Lego warships as well as hair raising carrier landings in the hallway), seated crew figures, simple assembly, AND I had just watched “Midway” in the theater with my dad when he got the kit for me. I think for the amount of pure fun and good memories, this kit wins hands down.
My favorite armor kit is the old Tamiya 1/35 Chieftain Mk.5, just because it looked so cool! I got one as a Christmas present from my mom in 1979, and have recently acquired another one to enjoy again, inaccuracies and all!
Despite being a sci-fi modeler, no specific kit jumps out from the pack. I suppose going with the enjoyment factor, it would be the MPC X-Wing. Me and my friends were so excited when the kit showed up at the LHS, and I remember us riding our bikes down to the store to get them. I built three in total.
So yeah, I sound like an old fart waxing nostalgic about out of date kits from a bygone era (each one of those kits has a high quality, modern equivalent). But it’s that spark of fun and excitement that made the hobby enjoyable. It’s kind of like playing cowboys as a kid versus actually working as a cowboy as an adult. It’s become more serious now, and thus a little less fun. But that’s a subject for another thread!
Oh and BTW, I confess to not playing “cowboys” as a kid - we played “spaceman”. I was a child born during the space race, and was lucky enough to have gained sufficient sentience to watch the moon landing and understand what it was about.