These are the best mashed potatoes that I've ever eaten!
There are the best mashed potatoes I've ever had!
These mashed potatoes are the best!
When referring to something as being the best, you are implying that this is of the most excellent type or quality that you have experienced. In this situation, it would mean that these mashed potatoes tops every other mashed potatoes that you've tasted in your life. The word 'tops' means to exceed an amount, or to be more than. Therefore, these mashed potatoes exceeds any other.
These are the best mashed potatoes that I've ever eaten.
These mashed potatoes are the best.
*These are the best mashed potatoes that I've ever eaten./These mashed potatoes are the best.- This means that you really enjoyed the mashed potatoes because you really enjoyed them and they are the more delicious than the ones you have tasted before.
例：These are the best mashed potatoes that I've ever eaten.
「このマッシュポテトは今まで食べた中で一番美味しい。」
例：These mashed potatoes are the best.
「このマッシュポテトは一番美味しい。」
これらの例文は、あなたがとても美味しいマッシュポテトを食べた時に使えるもので、「今まで食べた中で最高！」なことを表現できます。
These mashed potatoes are the best i have ever eaten!
These are the best mashed potatoes i've ever had!
These mashed poatoes are the best!
If you want to say that something is the best you have eaten you can say 'the best I have ever eaten', 'the best I've ever had' or 'are the best'
So you could say 'These mashed potatoes are the best I've ever eaten/I've ever had!'
The simplest way to say it is 'These mashed potatoes are the best!'
今まで食べた中で一番おいしいと言いたいなら、
'the best I have ever eaten'
'the best I've ever had'
または、
'are the best'
が使えます。
ですから、
'These mashed potatoes are the best I've ever eaten/I've ever had!'（このマッシュポテトは今まで食べた中で一番おいしいです）
と言えます。
一番簡単な言い方は、
'These mashed potatoes are the best!'（このマッシュポテトが一番おいしいです）
です。
They were the best mashed potatoes I have ever eaten
それらは私が今まで食べた中で最高のマッシュポテトでした
They were the best mashed potatoes I have ever eaten
The most delicious potatoes I've ever eaten
私が今まで食べた中で最もおいしいジャガイモ
You have enjoyed these mash potatoes the most out of all the mash potatoes you have eaten.
They were the best mashed potatoes I have ever eaten
（今まで食べたマッシュポテトで一番おいしかったです）
The most delicious potatoes I've ever eaten
（今まで食べたジャガイモで一番おいしかったです）
「今まで食べたどのマッシュポテトよりもそのマッシュポテトはおいしかった」ということです。
They were the best mashed potatoes I have ever eaten.
"They were the best that I have ever had." This explains that they taste better than any you have had before.
"They were the best mashed potatoes I have ever eaten." This is another way of explaining that the mashed potatoes were better than any others that you have eaten.
“They were the best that I have ever had.”
(それらは今まで食べた中で一番です。)
これは今まで食べたどのものよりも美味しいという意味です。
“They were the best mashed potatoes I have ever eaten.”
(それは今まで食べた中で一番のマッシュポテトです。)
これもまた、今まで食べたどのマッシュポテトよりも美味しいという意味です。
These are the best mashed potatoes I've ever tried!
These mashed potatoes are out of this world!
The two sentences you see provided above are excellent ways to express to your listener that the mashed potatoes you ate are the best mashed potatoes you have ever tried. In the second sentence you will see the term out of this world. This means something in incredible or amazing. This term is common in our everyday conversation and is appropriate for informal settings.
上記例文は二つとも、あなたが食べたマッシュポテトが今までで最高だったことを表す文章です。
二つ目の文章に、”out of this world"とういフレーズがありますが、これは「すごい・素晴らしい」という意味です。日常的に使える適切なフレーズです。
イギリス 
南アフリカ 
アメリカ合衆国 
