The period from 1955 (Grant Ave. on the A line) to 1968 (57th St. and Sixth Ave.) was the era of the small retangular station tiles. Then the MTA took over and things got even worse with the Broadway/Fourth Ave. BMT modifications. The pseudo-brick experiments at 49th St. and Broadway and Bowling Green came next -- some people don't like them but at least they're better than what had been done for the previous 20 years -- along with the partial blue tile job just north of BG on the 4/5, after which the traditional look finally returned.
The 57th and Sixth station qualifies as the most boring-looking station in Manhattan, IMHO, though other people may have different ideas.