What Megadeth Album to Get!?!?

I own "Rust in Peace" and "Peace Sells...But Who's Buyin?" and I love them both, I wanted to buy "Killing is my business" next but I don't find it anywhere, so I'm deciding between "Countdown to Extinction", "United Aboimnations" or "So Far, So Good... So What?". Could anybody give me some advice, Thanx.