It was X3, you can play as Zero, but only during the stages and not for Boss fights. Also if you died with him, he'd be gone for the rest of the game. If that happend, it was impossible to get the golden Armor for X.
Mega Man X and X2 were the two where Zero is a Non-Playable Character.
Personally, I didn't care much for X4. It was much too easy. X5 was a little more difficult, but it completely sacrificed plot. So, in my book, neither of them are very good. I personally liked the Original Mega Man games. Mega Man 4 was the first one I owned, and I bought Mega Man 2,3, and 6 shortly after. I never bought 5, because a friend had it, and I couldn't ever find the first one.
I also liked Mega Man 7 on the SNES, however, I haven't played it in a long time, so I can't say for certain I feel the same way. When the X series first came out, it was a big thing, at least, for me. It was cool because it was different from the same old Mega Man. Which, was basically just the same game repeatedly, spare the addition of Sliding and the Mega Buster, or the Rush armor-like jet suit in 6. I digress, what I was getting at was that the X series is no longer the different game series. It has become just like the origional, in the fact that it has repeated itself.
Mega Man Soccer, I didn't like it. Plain and simple. And this Mega Man Xtreme and what not, what a poor name. I'm sorry, but that's what I call catering to the lower mentality of society.
In conclusion of this jumbled mess, I am a fan of the Mega Man games. However, they're very repitious and lack origionality now days. The X series was at one point origional, but there is something as "Too much of a good thing." I do like X,X2, and X3. Not to mention the majority of the Rock Man series. The show wasn't anything special either. But, I won't go into that.