The correct phrase is "It tastes so good."
Here's why:
* "Tastes" is the present tense of the verb "to taste" and is used to describe something that is currently being tasted.
* "Tasted" is the past tense of "to taste" and would be used if the action of tasting has already happened.
Since you are describing something that is currently tasting good, you need to use the present tense verb "tastes."