普段よく寝れないけど昨晩はこの1ヶ月で一番よく寝れたよって英語でなんて言うの？

この場合、次のような言い方ができますよ。

ーI don't usually sleep that well, but last night was the best sleep I had in the last month.
「普段はあまりよく眠れないが、昨晩はここ１ヶ月で一番よく眠れた」
the best sleep I had in the last month「ここ１ヶ月で一番良い睡眠」

ーI never really have a good sleep, but last night was the first time I had a really good sleep for over a month.
「いつもよく眠れないんだけど、昨晩はここ１ヶ月あまりで初めてよく眠れた」
the first time I had a really good sleep for over a month「ここ１ヶ月あまりで良く眠れたのは初めて」

ご参考まで！

ご質問ありがとうございます。
・「I usually don't sleep well, but last night I had the best sleep I've had in a month.」

（意味) 普段よく寝れないけど昨晩はこの1ヶ月で一番よく寝れたよ

<例文>I usually don't sleep well, but last night I had the best sleep I've had in a month. I feel pretty good today.

<訳>普段よく寝れないけど昨晩はこの1ヶ月で一番よく寝れたよ。今日調子が結構いいです。

参考になれば幸いです。