@guinan @Trev @er1n s1 was when Roddenberry still played an active role... but he was clearly in his twilight years. It also suffered from early installment weirdness, as most shows do.
s2 was affected by the strike. Really bad. It was also when Crusher was introduced, that didn't help.
s3 saw the introduction of new writers. Roddenberry was basically gone.
Also, they kept going overbudget. They had bottle episodes to keep it cheap late-season (which often blew the budget anyway).