By top companies I'm assuming you mean in regards to quality, not just price. There are the usual PS favourite vendors like Brian Gavin, Whiteflash, Good Old Gold, High Performance Diamonds and James Allen. Someone more experienced can name another 3 for you
Depends on what you want. And what your requirements for 'top' companies.
The ones most recommended here for engagement rings are Brian Gavin, Good Old Gold, Whiteflash, James Allen, High Performance Diamonds, and Engagement Rings direct. That's only 6 but... those are really the only ones consistently recommended because they offer all the information needed for a knowledgeable purchase. Blue Nile does not offer this information, but they are sometimes recommended as well so I would put them 7th. But with the caveat that they are a vendor of last resort here on PS because they do not offer images, ASETs, or Idealscope images.