Gen 4/5 imo. I was still a kid and this is where I really started getting into the series on my own outside of just being into it because my older siblings liked it.
Got a lotta good memories with them, being completely blown away by the atmosphere of Black, being completely confused as to how the Entralink worked, feeling adventurous going through the Distortion world and defeating Cyrus, having tons of fun in the Wi-FI plazas, messing around with silly Action Replay codes, etc etc
I kinda regret not using the Dream World while it was still up. I didnt really have internet access over the week and I didn't really have my own computer to use it with, so I never really figured that stuff out. I hope people make a recreation service on Kaeru WFC at some point, though that sounds really difficult lol