The one I have the most ammo for, and the cheapest one to shoot. Any gun will sharpen your eye if you can shoot it enough. The cheapest one is the one to practice with. And any shooting is fun. Except the one that rattles my brains like the 54 Lyman GPR I had...that one hurt.
I've owned and shot many different guns.
But to stay on topic, the one that I found to be the most fun was a High Point 995 Carbine and the price of ammo (back then) wasn't too hateful. I would think that a Marlin Camp 9 would be even better but they're too pricey for my budget.
Currently, that's not even a 'firearm' for me - it's this multi-form BB gun!
When our grandsons and other youngsters visit, we set up a target stand in the backyard, and let them enjoy shooting in a safe, supervised environment. Our Umarex Morph 3X uses CO[SIZE=-1]2[/SIZE] cylinders to propel .177 caliber metal BBs (so it's not a firearm, per se). It can be set up four ways (hence the name change from 3X to 4X) for a variety of shooting experiences.
A distant 2nd is my Marlin Papoose because you get to assemble it every time you take it out of the bag.
I think I had more real 'fun' shooting my .22 cal Benjamin pump as a kid. The TC Hawken is great, except for the clean up. Busting clays and winged critters with a 12ga scattergun probably has more fond moments in the memory bank of great times with good friends and family.
The semi-auto "Commando", features parkerized metal and all wood stock, rear grip and horizontal forend in a durable black finish. With the same hard hitting .45 caliber, features and quality of our 1927 A1 deluxe model, it comes complete with a 30 round magazine and black nylon sling.
1st timer, so lets see how this works. My current favorite is a Marlin 30tk I picked up at a pawn shop last fall. After replacing the original sights with some XS sights, I cant miss with it. It is the coolest little gun I have ever owned. Kinda like the grandfather of the Guide Gun.