The level The Library in Halo CE gets a lot of flak for being "the worst level in Halo" - as in, all of Halo. All of the games.
I kinda love it, though?
Don't get me wrong, its not perfect. The level is dark. The environment is bland. It can sometimes be a little confusing to navigate. The floors are repeated again, and again. The same doors. The same scenery. The same tunnels. It also exclusively features the Flood as enemies, which aren't everybody's favourite CE enemy faction.
But it is a relentless assault on the player. In other levels in the game, there is breathing room. Places for the you to stop and reload. Places to take cover, and let your shields regenerate. The Flood doesn't let that happen, and they are appearing from every direction. In front of you. Behind you. From the pipes to your left, and to your right. The one you just killed is back alive again. You are constantly shooting, moving, finding the next target, scavenging ammo, trying to buy yourself enough time to regenerate your shields, and load as many shells into your shotgun as you can, before the Flood come for you again. It is really stressful, particularly on the harder difficulties.
Sure, it may not be the most imaginative level, but its gameplay is matched only by the (equally hated) Cortana level from Halo 3, and I adore them both.
So yeah, The Library is pretty cool.