This is the 3358th most frequent Japanese word.
最上
"最上" means "the best" or "highest."
In this sentence, '最上' is used as an adjective meaning 'the best' to describe the noun '天気' (weather).
最上の天気ですね。
It's the best weather, isn't it?
Here, '最上' is used as a noun meaning 'top quality' or 'superb' within the context of describing the cuisine.
このレストランの料理は最上です。
This restaurant's cuisine is superb.
In this sentence, '最上級' incorporates '最上' as a prefix to mean 'the utmost level' relating to '快適さ' (comfort).