Mass Effect for lore, easily. Although Halo's lore might be more in-depth, Mass Effect provided the Codex, so players didn't feel like they were missing out if they didn't read the books. The codex was also built by the player, so you couldn't just read through it and know all the lore before getting past the first mission; you had to ask questions to learn. Although this made Shepard at times, seem like he had just been dropped into the galaxy with absolutely no prior knowledge of anything, this was a risk that paid off.