Halo1's were alright, but people usually enjoy the originals simply because H1 was the first Halo/Nostalgia. Not to say there weren't maps that were great. Timberland and Sidewinder are quick reminders of good H1 maps.

Halo3's outside maps were the best ones out of all of them in my opinion. Bungie just does a great job at capturing the wide-open environments. I don't count Isolation as open though, if you know what I mean.