iMovie will run on just about any Mac that is in the current Apple line.
The best are the iMacs. Any model will do, but the 27" models are really good for editing.
You can get the i5 processor or the i7 processor. The i7 is faster, but both work fine for iMovie.
Memory can really help. iMovie can happily use 8GB, and I currently am using 16GB.
Next would be the MacBook Pros. Lots of people are happily editing movies on MacBook Pros.
Finally, the MacBook Airs. iMovie will run on a MacBook Air, but they are not great for movies because they do not have a hard drive. They have a small solid state drive rather than a hard drive, and the solid state drives are generally too small for video files, which can be quite large.
With any Mac you get, if you plan to do a lot of movies, it is good to get external disk drives. These days, you can get USB3 or Thunderbolt drives. The USB3 ones are pretty inexpensive. The Thunderbolt Drives are much faster, but you do not really need their speed unless you move to Final Cut Pro X and are shooting with multiple cameras.