(I'm still sick. But I couldn't take being in bed anymore, so I'm going to attempt a proper post this morning).
I'll just bet you thought I'd say Champions or the HERO System. Not gonna. While I freely admit the system has its complexities, they are almost entirely front-loaded and rigorously consistent. By this I mean that once you've gone through the exercise of putting a character together, you know more or less precisely what the character can do, and the actual system at the table remains the same, whether you shift out to the macro-level, or drill down to a ridiculous level of detail.
Compared to exception-based games like D&D or Exalted, I find HERO pretty easy, truth be told. Besides, there are plenty of games out there that leave Champions sucking fumes when it comes to complexity. Space Opera and Aftermath come immediately to mind, as does Living Steel. (True Fact: I once played a man-to-man combat in LS with an experienced GM. Three hours later, I still had no idea what I was doing.)
Thing is, I don't actually own those games, so I can't really cite them, if I'm being honest. Thankfully, I have one that stands the test of time in its complexity through three editions: Chivalry & Sorcery.
One of the first D&D alternates, C&S started out insanely complex, doubled down, and then went off in a nigh-incomprehensible third edition. Here's the second edition character sheet. Well, part of it anyway.
If you're playing a magician, expect a few more pages. Also, some very heavy lifting in the math department. And that's just to figure out your CHANCE of casting a spell. Seriously, it's mind-boggling.