I believe the most natural ways to phrase your sentence are as follows:
( 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.
Today the most difficult part was Yoga.
Note: の can be replaced with こと, they are interchangeable.
( 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".
He is one of the most intelligent students in his class.