I liked MM 1-8 a lot. I played them relentlessly on the Anniversary Collection. When MM9 came along I had to get it. When I played it I died about 50 times but it was still fun.
X 1-6 were great but I felt 6 was a bit rushed. My favorite was X 3. I haven't played X7 or 8 so I don't have anything to say about those. The Battle Network Series is absolutely terrible. I only have fun Playing BN 2. I HAD 6 but sold it because it wasn't fun.
The Zero series are awesome and kinda hard sometimes. I still haven't beat 2. I love them and I play them a lot. Mega man and bass is the toughest though IMO. I'm not even gonna bother with Star Force.
Mega man legends/Megaman 64 1 was a fun game and although I never played 2 I would gladly buy a 3rd.