Brock Boeser
While it would have been great to get a Mikko Rantanen, it didn't seem like it was going to be truly realistic. On top of what the Kings would have had to trade to the Hurricanes, which would have been a lot, they would have also needed to get a contract extension finalized with Rantanen. While the Stars were able to do that, it sounds like Rantanen was very selective in terms of who he would sign an extension with and the Kings were not one of those teams.
If the Kings wanted to make a big swing at the trade deadline, the best option would have been Brock Boeser, however it would have come with its own challenges and hurdles. The first would be similar to Rantanen, that the Kings would have wanted to have confidence in that they could sign him to an extension as he is set to be a free agent this summer.
In the case they could do that, they would have had to make it enticing enough for the Vancouver Canucks to trade one of their top players to a division rival who they are competing for a playoff spot with. If the Canucks were ready to give up on the season, it would have been an easier sell.
After the NHL Trade Deadline, it was reported by Elliotte Friedman that the Canucks were asking for a first round pick. If that was the starting point, the Kings should have been happy to offer that and another prospect or two to get the deal done. He is an excellent player who could have been a top-line player that would help transform this offense, which has had its struggles.