Fun fact: 9/10 users with no linked gamertag are better than you at Halo. That doesn't necessarily mean that I am; but it does mean there's a pretty solid chance.
I quite like Mt. Lam Lam. inb4 hate.
Synapse is okay, as is Asylum. Most of the maps in MM are decidedly average, Forge-made or otherwise. There aren't really any gems like there were in previous Halos.