1) There are a ton of great forge objects but my favorite would proberly be the Dominion and Extraction objects found in Halo4. Unlike the other objects in Forge, these simple props have proven themselves to be far more useful to me. Dominion Objects are great for filling in gaps, creating aesthetic items like shelves/walls, and aslo double as player controlable features for a map. Base Shields cause almost no lag, are fully color controlled, and can be set to only allow a certain team to pass through. Dominion Turrets and Vehicle Pads prevent vehicle theft and provide control points for teams. Extraction Objects are a lot fewer in number but also provide a great set of uses. They cost a little more but are also plentiful in number. Because of this they can be used as walls and doors. Their team color controls allows Forgers to create call outs and help with map navigation. Unlike other Forge objects, Extraction items literally glows bright with team colors. These objects also are destroyable with a respawn timer. This allows players cover while at the same time making custom maps interactive.

2) My favorite two Forge maps are Halo Reach's Forge World and Halo4's Forge Island. However both have their flaws and I can only choose one. Because of that I must choose Forge Island. True the object pallet is all screwed up and there is a LOT less solid ground to work with. But it has three features that the others don't.
Trees, Space, and the ability to travel into it's environment and forge freely. That plus it doesn't have Gravemind's Death Race. Which is a huge plus. ;)

3) Three NEW changes I'd like to see.
A- Sizable/Mirrorable objects. Having to use multiple blocks to fill a space blows. As does not being able to objects on opposite sides of a symmetrical map because they don't face the correct direction. (I'm looking at you Staircase and Buildings)
B- Custom Magnet Points. This would mean we are able to turn on and off which magnetic points of a object used through the object's submenu. We are also able to move said points along the object's edges and corners.
C- I was going to suggest finer controls as we saw in Reach. But that's not new and may return anyway. Instead I'd like to see Controllable Environmental Effects. Snow, rain, fog, wind, splentering trees, breaking glass, mix of those, etc. But this could also include man made events like explosions, trees and structures falling over, buildings/cover breaking apart, etc. And don't forget about the animals and other beasts. Nothing is scarier then chasing another player around a corner and suddenly finding yourself fighting off a Alien beast. And nothing's funnier then seeing your enemy run back toward you with said beast chasing him. Not all beast have to be violent either.

That is all.