I'm a fairly avid gamer and could rant about the massive spread of game titles I've played, but in terms of Ys I've played 1,2, 3 (in the form of Oath), 6 and Origins. 6 is my favourite, but Oath wins the soudtrack contest.
Welcome! Nice, it's not every Ys fan that regards Ys VI as his or her favorite, that's neat! I personally still prefer Ys II at least, and possibly also Ys I and/or IV, but Ys VI is legendary for me regardlessly, as well. My personal favorite if we are to count all Ys games released after the mid 90s up to this day.