BTW major spoilers from the latest Arrow episode and the Walking Dead so if you don’t want any spoilers stop reading now.

Felicity Smoak

Still makes the most sense to me as her stories are simply running out and it would have the most impact on Oliver's character.

So I’ve been trying to figure out who could have died in that explosion based on confirmed plot points and dialogue in the episode and honestly, I still think it makes sense that it be Felicity as she simply has no more use anymore for the story beyond her tech skills alone. We know pretty much all of her background and her present day stories are fairly boring. At the same time I thought it could be Samantha but I really hoped it wouldn’t be her so the Arrow writers won't have some stupid Felicity step-mother story. But then it dawned on me: Could Felicity and Samantha have both died in the explosion? It’s clear now that Oliver is going to be involved in William’s life now that he knows who he is and his relation to him so that would imply Samantha will die but I really think Adrian’s words “You’re one loss away from getting destroyed” have to come somewhat true to test Oliver’s resolve as a parent so at least one major player has to die, and in a way that will really affect him. Adrian also mentioned Felicity quite often when speaking of whose death he thinks would destroy him. In fact his final words where his belief that Samantha and Felicity were about to die.

Samantha Clayton

This is an equally logical assumption, so that's why I think she and Felicity both just died.

If Felicity and Samantha both just died then you’d have Oliver and William both experiencing a devastating loss in the next season. It would be particularly challenging for Oliver because his son will have just lost his mother, the only family he's known, and will be struggling with his grief. Because of this Oliver will have to be there for William and be the parent he needs him to be without letting his own grief for Felicity's death get in the way. After all, and I think this goes without saying, a parent's job is to be there for their children when they need them without letting their emotions get in the way. Sad as it sounds Oliver would basically need to bench his grief for losing Felicity to focus on the well being of his son, as it's what Samantha would expect of him and correctly so. 

William Clayton

If William just lost his mother I think it would be great story material if Oliver just lost Felicity, and he has to bury that grief to be the parent William needs him to be.

On top of this Oliver would need to manage being Green Arrow whilst being a parent while trying to fight the grief of losing one of the loves of his life. Having Samantha die could also motivate Rene more about being more involved in his daughter’s life and it could inspire Slade to find Joe and that gives him an excuse to leave without dying. Also that talk between Oliver and Felicity before their kiss (ugh!) to me just screamed that she is gonna die because they just made up. Also as I’ve been pointing out this entire episode was basically Walking Dead 6x16 with Negan’s victim being known until 7x01 and then bang not one but two main characters were killed! Who says they’re not doing the same thing on Arrow? Before the episode was released I narrowed the choices down to Glenn and Abraham based on the way the story was going and the continued use of their characters but I couldn't figure out which one it would be, but I knew it would be one of them. Turns out I actually had the answer already when I narrowed it down to them both. Well now I'm getting the same vibe from this ending.

Felicity shot

I seriously got these vibes from the last episode, and last time I got vibes like this I correctly guessed that Abraham would die in TWD!

I know Felicity is still considered main cast but that doesn’t mean she’ll be alive. It could be hallucinations, side-effects of Oliver's grief while raising William, or flashbacks to previous seasons which they did mention they’ll do next season. Or she’ll simply be credited as main cast to cover for her death. They have also said things like "she'll be back" without specifying how long. I also know about the Helix bulls*** which will likely come back into play but honestly they can just use Curtis for that, in fact I’d like that much better if this Cayden James prick is the villain. While it would be suck if Samantha died regardless I’d be quite frustrated if Prometheus didn’t just kill a main character, given all the build up all season. 

So this is my opinion on who just died in the Arrow finale and I hope I'm right.