I can only speak for myself, but it had a great start and only declined in seasons 2 and three, with multiple outright problematic episodes in season 3 (that also just went against everything Star Trek represents in terms of values and philosophy, and while there occasionally were really troublesome episodes before (Code of Honor, anyone?), the density of those episodes in S3 was awfully high). Season 4 was slightly better again, but some issues seem to have become woven into the fabric of the show (why on earth are they trying to depict Spock so damn straight and take all the queerness out of him?), the amount of gimmick episodes was staggering and I do not believe they can reach the highs of S1 again. And when a show peaks in its first season, I find that pretty sad, tbh (even more so the longer a show runs).