Planetside (the original) - in my opinion, in it's day it was massively overlooked and WAY ahead of its time. (but its steep learning curve and monthly subscription probably didn't help to draw new players in). It was an amazing game for its time - the servers don't run anymore, so it's not really something accessible now, so I'm unsure if it could pass the criteria of regularly being re-played tho...
The game came out in 2003, had 3 teams/empires each map could hold up to 166 players (later dropped to 133 to help populate multiple servers) for each side. So, a fps shooter with a 498 (later 399) player servers!!! Not just a fps, but also 12 different common ground vehicles (plus an additional 4 specific to each empire/team), then 7 different aircraft. I think there were approx 10 big maps (plus smaller ones), loads of weapons, skills, abilities etc. Compared with modern shooters, the only thing really missing was location damage. Even today, 18.5 years later, something recent like Battlefield 2042 brags about only 128 players (and nowhere near the weapons, vehicles or customisation).