Completing the Vidmaster challenges in Halo 3 and ODST to earn Recon armor is probably up there for me. They sure didn’t make it easy and it feels good to have earned Recon. For example this achievement:
Vidmaster Challenge: Endure: In Firefight, on any mission, pass the 4th set on 4-player Heroic LIVE co-op.
This one alone takes nearly 3 hours of continuous playtime per attempt and I remember the frustration if you failed a wave and had to start over.
Everyone wanted Recon so it really brought people together to grind out the challenges. Man I really miss the old Bungie and those Halo 3 days on Xbox 360.