I hit 100k with “A Monument To All Your Sins” in Halo Reach, but aside from that, I haven’t used any fitting achievements for other milestones. Instead, I try to line up a game completion with each milestone. 200k was the completion of Binary Domain, and 300k was Halo 4 (using that solo legendary achievement as well).
Now to decide what I want to do for 400k. Maybe Halo 5?