My proudest Halo achievement is definitely completing all of the MCC 3 Hour Legendary Speedrunning Achievements for each Halo game. Halo 2, more specifically, as that is easily the hardest speedrun and I managed to do it solo. The other games, however, I completed with friends as I had already proved to myself I could do them solo considering I completed the hardest one by myself. Some of which only took one playthrough and only a couple of re-dos. Halo: Reach, when it first released on MCC, I played through solo on legendary in about 4.5 hours. ODST and 4 were the same. The other games' speedrunning achievements were definitely more challenging but nowhere near as bad as Halo 2. I proudly boast the Platinum Halo 2 Nameplate. I wish it looked as cool as Reach's 3 Hour nameplate, though.
For anyone else who wants to complete these achievements, you should view guides made by Nak3d Eli on YouTube. Without those guides, I would not have completed most of the games' speedrunning achievements (mainly Halo 2's). Don't get me wrong, though, even with these guides, the speedruns will still take tons of practice, trial-and-error, and even some luck. With enough dedication and skill, however, they can be completed. Good luck to anyone who wishes to give these achievements a shot!
Nak3d Eli's Legendary Speedrunning Playlist: https://www.youtube.com/playlist?list=PL4RoBZrNqFcoc0lcgUrlG5aS7UgxIy-9e