Oliver Queen loved Sara Lance more than all the other women in his life and we were shown that from the pilot on, here is why:
Sara was the first woman we knew Oliver was involved with, the first woman we ever saw him with.
Immature, irresponsible, selfish, "I detest swimming" playboy Ollie was willing to jump right back into the freezing waters of the North China Sea in the middle of a storm to go look for Sara. His father had to literally hold him back, to keep him from throwing himself back into the water to go look for her.
He talked about Laurel on the island but not Sara. Never Sara. And we know Oliver doesn't like to share his feelings and talk about his emotions. If something's too painful he doesn't talk about it.
Sara was the very first person he was willing to die and kill for. And I'm talking about make the choice and kill someone in cold blood.
He told Laurel he thinks about her every day.
He got up and prematurely ended the lie detector test he insisted on taking after bringing up Sara. He completely shut down.
He kept his promise to her and lied to her family which had them hate him, while the truth might have helped mend those relationships some. Even while thinking she was dead, after she came back home and even after her second death. He still insists that "Sara had her own secrets and those weren't his to tell."
He killed Ivo so she didn't have to, so she wouldn't change like he had after killing someone. This was the first time Oliver killed and it wasn't in self-defense or affect because someone he cared about was in imminent danger. There was no need for him to kill Ivo, yet he did. Simply to preserve the person Sara was. It was her promise, a choice he had let her make, yet, he fullfilled that promise in her stead to spare her the pain of knowing what it felt like to take someone's life, after he had fought her when she told him he had to kill Ivo before he could talk to Slade. Oliver was vehemently against killing someone in cold blood until it was to keep Sara from doing it. If that isn't love. I don't know what is.
They spent months on Lian Yu together were they had to get to know each other again and rebuild their trust after Sara betrayed him but Oliver still saved and protected her, even with Shado and Slade's interjections.
While I do not believe that Oliver choose Sara over Shado, Ivo made the choice for him, he'd seen how protective Oliver had been of both women and concluded Oliver would get between whichever one he was pointing the gun at and the gun. Ivo decided that Sara would live. That was the one time he didn't chose Sara. But he chose her over Laurel when begging her to come on the Gambit with him, a trip that took about three weeks per way. He chose her over Laurel when she came back home. He chose her on Lian Yu when risking everything for her several times and doing what he could to protect her and keep her safe. Whenever there was a choice and Sara was one of the options he chose Sara.
Sara was the first woman Oliver was willing to open up and fully commit to. He let her know he was scared because of Slade and didn't know what to do. He bared himself to her and showed her his weakness. Something Oliver struggled doing all throughout the show. Oliver asked Sara to move in with him, which was very telling especially because that had been a point of major dissent between him and Laurel and one of the main reasons he joined his father on the Gambit to begin with.
After Sara died he told Felicity "if I grieve, no one else gets to." Which means he would completely fall apart and most likely do what he had done after Tommy's death. Run and spend months in seclussion.
He saw Sara as a part of his family. And just because there are people claiming they didn't spend time with each other besides the island; if Oliver and Laurel had known each other forever, so did Oliver and Sara. No one can tell me party boy Oliver didn't hang with party girl Sara. They've known each other forever, had similar interests and character, they'd for sure gone out partying together before the Gambit. Their affair had to have started somehow and it wasn't on the Gambit, Oliver made that clear while talking to Helena "I started sleeping with Sara, I started sleeping with my girlfriend's sister".
Oliver put a tracker on Sara because "I couldn't lose you again".
The moment Sara stepped back into his life, she was all he cared about. And he did whatever he could to keep her in his life.
Bringing Sara up was a sure way to get an emotional response from him.
He always dropped everything if she was in danger. Be it on the freighter, Lian Yu getting kidnapped by Slade, because of the League, soulless, lost in time with the Legends, abducted by aliens, nearly killed by a Nazi Supergirl... if Sara was in danger, Oliver would push through his own pain in order to get to her.
Sara always managed to get him out of his own head and push him on a better path, she always believed in him. As was shown in Star City 2046. Sara reached him when no one else could.
They didn't need words to communicate. One look was enough.
They had fun with each other.
Sara never put him down or held his past against him. She didn't use emotional blackmail to have him fall in line. She didn't expect him to just get over his trauma.
Her death pushed him over the edge and down a path of self-destruction. The look over utter and complete devastation on his face when he saw her dead body and the fact that he went into shock and spent the entire episode desperately trying to not fall apart.
Sara showed back up in Starling, and Oliver distanced himself from Felicity. Plus, the smile of pure joy on his face when she implied that she might be around more again.
Even Slade understood what Sara meant to him, he'd seen first hand how far Oliver had been willing to go for her. "... someone who means everything to you..."
While both Oliver and Sara felt guilt about the way they ended up on the Gambit together, neither regrets that choice.
Sara was with him both times he died.
He respected her choice to leave.
And most importantly it is a fact that Oliver loved Sara, because he said so himself: "a woman that I loved was shot full of arrows and sent tumbling off a roofop." Oliver loved Sara, it's a fact that we know because Oliver told us so. It boogles my mind how so many people claim he never actually loved her. Not sure what their definition of love is.
Sara Lance was the love of Oliver's life. Outside circumstances always got in their way. The main issue they had was timing, nothing else. Their feelings were strong and stayed that way throughout both shows.
And last of all, there would be no Green Arrow without Sara. Sara put Oliver on the path of becoming the man he turned out to be, just like Oliver put Sara on the path of becoming the woman she turned out to be. They were soulmates and their lives were intristically bound together.
Their lives paralled each others and in the end they both died. They had the tragic star crossed love story. They suffered so much just to die young in very unfair ways. Had they stuck together they'd probably still be alive.
And yes, Sara is dead. She was killed and a clone alien hypride is not Sara. It's an imposter mascarading as her. Oliver and Sara are probably hanging out with Tommy and Laurel, enjoying afterlife together. (I refuse to acknowledge the stupid "everyone but Laurel is alive again" plot they shoved down out throats)
Sara was the one that got away. It was always her choice to leave. He always wanted her to stay.
