For me, it has always been the megaman battle network series (3 being my favorite story-wise and 6 in terms of game mechanics).
They never fail to bring me back to my childhood, the wonders of the internet’s potential and so much roleplay potential.
How about you?
The zero games are difficult for sure. The only ones I finished without cheats were megaman zero 3 and 4.
I liked them all. And I liked the difficulty. The first one is just hard to go back to and replay after the QoL changes from 2 onward.