My favorite is Casino for three reasons:
1. It's a technical marvel: production design, costuming, cinematography and lighting. It's almost dreamlike, and really makes 1970s Vegas look like some lost golden age.
2. Storywise, I thought that it was more of a Shakespearean tragedy than anything else, which gave it more of a plot than Goodfellas. Ace, the "hero", brought down by his fatal flaw: his obsession with controlling the uncontrollable.
3. Finally, Pesci. While Pesci is at most a supporting actor in Goodfellas, an adjunct to the Liotta/De Niro dynamic, he is every bit De Niro's equal in Casino. It's a testament to his talent that his performance never becomes grating, and always remains interesting.