I, personally, would have a hard time deciding whether 2 or 3 is the best. I love both of them.
2's graphics appeal more to me, and i think it had just a tad bit more atmosphere around it, while 3 is a great follow-up, building up on the mechanics from the previous two parts, adding more content, and keeping true to the series.
4 is not bad, but the combat system is shit, and i haven't played 5 yet.
To be perfectly honest, i wouldn't cry if one would be picked over the other. As long as one of them would be there...