.223 for sure. Guns, you can go from a used Stevens around $300-350 all the way up to thou$and$. Brass, from cheap used off the EE to Lapua around $80/100, with choices in between. Bullets, depends on twist, from light 30-35gr varmint bullets all the way up to 90's for targets and long range. Can be as cheap or expensive as you want, is pretty easy to load for, factory ammo is about as cheap as you'll find in a centerfire short of milsurp in other calibers.
I had a .204, more of a barrel burner, for me was fussier to load for, for sure a lot more cleaning of the barrel...on the other hand seeing a number that started with a '4' on the chrony was kinda cool. Got rid of it, though wouldn't totally rule out owning one again.
I have a .221 Fireball incoming, that I'm really excited to try out. .223 case cut down by around 1/3, no recoil, less powder usage, somewhat more limited on bullet selection (say, 50gr max as is 1/14 twist), but still 85-90% of the speed of a .223. CZ makes em in their 527 varmint series, last I looked Clay had one in stock if you want to try something a little more esoteric. I even know where to find some brass in stock right now, I am led to understand it can be a bit hard to come by as only Rem makes it factory.
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