I hate summer the most. The thing I hate the second most is allergy season.
Summer is my least favourite season and after that it's allergy time.
この場合、次のような言い方ができますよ。
ーI hate summer the most. The thing I hate the second most is allergy season.
「夏が一番嫌い。2番目に嫌いなのはアレルギーの季節」
to hate the most で「一番嫌い」と言えます。
ーSummer is my least favourite season and after that it's allergy time.
「夏は私の一番嫌いな季節で、その次がアレルギーの時期です」
one's least favourite season で「一番嫌いな季節」とも言えます。
ご参考まで!