Yeah, I tend to agree. While Foundation, Foundation & Empire, and Second Foundation were pretty good (still waiting for those atomic ashtrays that distintegrate stuff dropped in them down to their atoms), the pre-prequels & se-sequels left something to be desired, esp. when he tried to harmonize the I, Robot universe with the Foundation universe. IIRC Daneel Olivaw (original robot) turned out to be the 2nd Foundation mastermind, steering humanity on its ultimate course based on the zeroth law. If he had included any hint of that in either of the original series, it would have come off much better.
He was my second favorite classic sci-fi author, after Arthur C. Clarke. While Asimov told his stories mainly through character interactions & dialog, Clarke could paint you a picture and immerse you headfirst into his alien worlds through sheer imagery.
However Asimov's Nightfall, End of Eternity, The Gods Themselves and I forget the title of what I believe is one of the shortest sci-fi short stories ever (where as the last star in the universe burns out, the final human programs the ultimate computer to reverse entropy, the computer computes for endless millenia and then finally outputs "Let there be light!"), arguably place him as #1.
And Robert Heinlein (before he started writing about sex with his mother) was excellent too, particularly the time-travel story where the protagonist meets himself about half a dozen times, beating himself up several times, etc..
On a more recent note, I can recommend the Gaea trilogy by John Varley - Titan, Wizard & Demon.