My favorite is season two. The first season was too broad for my taste. (Though Lucy and her Vitametavegamin bit remains, imo, the very best thing Lucille Ball ever did. My gosh, her sense of timing was unparalleled in that scene. Just brilliant.) And that first year could find its origins in vaudeville, which is too old school for my taste.
The second sense, getting off to a rip roaring start with "Job Switching", found Ball and the scripts consistently fresh and bright. It must have all still been very exciting and there are plenty laughs here. Plus, and this is just an aside, but Lucy was pregnant this season and the extra baby fat made her quite adorable. She was a real babyface here and this suited her childlike character perfectly.
Third season is also excellent and I love the first half of the fourth season culminating in the hilarious "Ethel's Birthday". There were a few quite good episodes after that and then they got to Hollywood and, for me, things went downhill fast. Mischievous and endearing Lucy turned into selfish and overbearing Lucy overnight.
I will say that in the episode "California, Here We Come!" when the gang is driving over the Brooklyn bridge (in their baby blue Cadillac convertible, no less!) singing the title song - a real sweet, happy little tune - I always tear up. Those 90 seconds pretty much portray the essence of the show's likability. Awwww... Pure sentiment.