I believe the most natural ways to phrase your sentence are as follows:
今日はヨガが一番難しかった(です)。
( Out of all the activities I did today, ) Yoga was the most difficult.
Note: emphasis is on the word "yoga". です can be added but is not necessary.
今日一番難しかったのはヨガでした。
Today the most difficult part was Yoga.
Note: の can be replaced with こと, they are interchangeable.
今日のヨガが一番難しかった(です)。
( Out of all the yoga I've done, ) Today's yoga was the most difficult.
Note: emphasis is on the word "today".
In general, 一番 and 最も are interchangeable, however 一番 indicates an explicit number one ranking, whereas 最も does not necessarily indicate a number one ranking when referring to a group, just like the word "most".
彼はクラスで一番頭のいい生徒の一人です。✘
彼はクラスで最も頭のいい生徒の一人です。✔
He is one of the most intelligent students in his class.