In my opinion Cata was better than Mop by a mile.
What was better?
- 5 mans, lots of variety and they actually offered some challenge at start.
- Valor system was better.
- Raiding were simpler and you could enjoy them more, instead as of now there are too many tactics to follow which kills the fun aspect.
- Alt's were easier and actually fun to gear up.
- There wasn't "must do" dailies.
Im sure i missed some, but i for one enjoyed Cata lot more than i do with the Mop.
For me i belive it would go like TBC > Wotlk > Cata > Vanilla > Mop