Ja lubię (this is the) best thing since sliced bread :D
Should it be "...the best thing since sliced bread" or "...best thing since sliced bread"? Which of the two do you like best? (or ...the best?).
Be brave. Say what you like best (or the best?), sliced bread or anything else.
You should rather use 'This is the best thing...'.
If you use it for example in a casual situation you can omit it, for example: 'Wow! Best thing since sliced bread!' ;P
Whether it is question about what someone likes the most, or just statement, it should be: "what (do) you like best (?)" (question) :)