My favorite show ever. Like many, if I had to pick, I'd say I liked the early episodes with Blake and Trapper because those eps were a little more irreverent and were just funnier. However, I found that I'll enjoy probably about 95 to 98 percent of the episodes throughout its run.
To me, the cutoff is Frank Burns. I loved him as a foil for Hawkeye and Trapper/BJ, and I thought the show was a bit poorer for his absence. I grew to tolerate Winchester, and he did have his moments. And I understood what the producers were going for: Instead of bringing in a copy of the Burns character, who was regular Army and totally incompentent, they brought in a character in Charles, who was a brilliant surgeon (and arrogantly so) and also shared a lot of Hawkeye's and BJ's scorn for the way the Army did things.
Otherwise, I always loved the Potter and Hunnicut characters, and one of things I always liked was seeing Klinger evolve from a one-note joke to a guy who grew into the company clerk's job.