It's less polished, narratively; it's more experimental, more improvised, more closer to home. Its mastery is in its evocation of time and place - soundtrack, mise en scene, acting. It's as aesthetically innovative as all the films you mention, Irish.

Casino is two hours exposition and one hour wrap-up - and very effective as such, but hardly worth pushing up against his others; The Departed is an exciting and solid but relatively tame remake in comparison to the vicious and humane Mean Streets.

I'd rank his top five thus:

1. Taxi Driver
2. GoodFellas
3. Mean Streets
4. Raging Bull
5. King of Comedy

And the only other of his works which might be as excellent as those is Cape Fear. Most of his others are great-not-masterpiece efforts.