I’m not a huge fan of complex games systems, I’ve played a few and I tend to find that they get in the way of the game. As with a great many in the roleplaying community, I’ve moved towards a preference for rules light systems that favour flexibility over rigid details that cover every minor occurrence in a game.
In terms of games that I considered complex at the time, it’d be AD&D 2nd ed. It was the first rpg that I properly played, the first I ran, the first I learnt and given that I was in my early teens at the time, it was a pretty damn complex game but it was also the game that set me on the path i’m on today, decades later.
For games that at truly complex, maybe Shadowrun, 3rd is probably the most complicated i’ve ever gotten properly to grips with. Shadowrun is one of those systems that has rules that cover everything, including the infamous chunky salsa rule for rebounding shockwaves from explosions. If you ever want an interesting challenge, check out the various rules for building and creating your own cyberdeck and programs in Virtual Realities 2.0.
The most complex i’ve read and tried to get to grips wth is Alpha Omega. That game has something like 12 modifiers that apply to EVERY SINGLE COMBAT ROLL, including, attacker’s stance (lying down, crawling, crouching ducking, standing), defender’s stance, attacker’s movement speed, defender’s movement speed, relative distance between attacker and defender, cover, concealment, lighting, distance between attacker and defender and more. That’s all before you get to monstrosity that is it’s magic system.