X series is the best one, with X5 being my favorite title. It had the best sound track, ability swap characters, multiple armors for X, enhancements for both characters, and it had one of my favorite boss fights. Yeah, I'd agree with X having the highest highs and lowest lows (X7).
Zero series comes in second place. It's the most consistent series, but it comes in due second place due to me preferring the art style in the X series over the Zero series. And in the earlier games, I wasn't fan of leveling up the weapons nor getting a high rank to unlock skills.
ZX is in third since it's map system is awful, but the combat system was still the same as Zero. Trying to copy one of the worst series (Metroid) was a mistake, there's a reason Metorid why now the ZX series is dead and left in a cliff hanger. No one wants Metroids (that's why Metorid is also dead.)
Battle Network comes in fourth and Starforce in fifth. The games were enjoyable, but I just prefer thegameplay from the X/Zero/ZX series.
Classic series is boring/slow paced, and I never played Legends since it looks janky and ugly.