Yeah never really gotten the fuss. It's a fairly bog-standard thriller that's masking as something more with the whole "dream inside a dream" thing. I'll maybe need to rewatch it at some point but I don't remember ever thinking there was much depth within it.
As for his best, from what I've seen it'd be Dunkirk or TDK; the former's probably his most mature film I've seen but TDK is incredibly entertaining and probably the best film within that sort of comic book sub-genre. I've not seen his earlier work though so wouldn't be able to comment fully.
The level of complexity in Inception is far more impressive than Dunkirk, Interstellar, or the like.