I think powerhouse, sword base, countdown, zealot and boardwalk are all good maps. They're certainly not the best of Halo - but they've grown on me. I'd remove the low grav area on zealot, the shrubs/trees on boardwalk, the tiny lift room in sword base and the hammer in powerhouse - in a heartbeat. But otherwise they're all really fun maps with lots of options.
The forgeworld maps vary, but Aslyum, precipice, kingdom, treasury, enclosed and uncaged are all really enjoyable maps. I think Enclosed ranks somewhere among the best Halo maps in the series. I think the only awful forgeworld maps are pinnacle, think twice and affinity. While certainly playable they tend to turn into power weapon domination matches - and they all have 'ideal' areas for the winning team to hold - it makes many areas of the map fairly worthless or overly dangerous.
As for the Halo MP Rankings:
Halo 2 before modding > Reach > Halo 3 > CE > ODST > Wars > Halo 2 during modding.
Reach is nice. Very nice.