My all time personal favourite is the Reach-Hangemhigh final, with ai bots
The best thing is they have 150% more shield and hp in every settings, pistol is a little less powerful, and they will haunt you even in your dreams with their swords.. not to talk about sniper and dmr ones, they will just 2-3 sk you anytime you face them.. Especially good for tactics, aim and reflex improvement
Also, they move so randomly, I was at the blue base near the flag, started shooting one of them, the next time I found him he was camping near the tombstones at the blue base
But if we count modified single player maps as custom ones, my second fav is clearly the H3 version of the Two Betrayals.. Used to play it twice a week on legendary, now I fvcked up my halo, and can't really play it