Definitely 9, 11, and then 10.
When Seymour debuted he was more of an affable "mad scientist" type. He later became the "snotty headmaster" type.
The earliest Seymour episodes, fromWhy Does Norman Clegg Buy Ladies Elastic Stockings? to Masculine Purse, is an absolutely special time for Summer Wine. However, I feel the Set the People Free is the best Seymour episode and perhaps the quintessential episode for Summer Wine's theme of carefree single men and oppressed married ones!
Season 11 has some real gems too, such as Three Men and A Mangle.