Considering that I don't know how fire would enter a fight with Buzzy or Koopa (btw, it's Beetle, not Beatle), since neither spits fire, a fire-proof shell wouldn't mean much to Buzzy.
But Koopas have evolved over the years compared to Buzzy not really evolving much as far as I can tell, and although I'm not sure Koopas have grown any SMARTER since then, I'd say Koopa Troopa would win.
Besides, I like Koopa more.