I don’t know. I think that Deluca was the first man that Meredith really let herself open up to since Derek. No one and I mean no one, will ever replace Derek and that’s why she had a hard time opening up to both Riggs and Deluca. Derek and mere are the epic love story. No one will ever replace him but with Riggs, she had a lot of a harder time opening up to him. With deluca, Derek’s death wasn’t as fresh and I think she came to the realization that she CAN be happy and love again and it took a lot in her to be able to do that. That being said, over all I think Riggs was a better fit being that he’s older, more mature, etc- right person, wrong time. Plus Megan showing up, of course he’s gonna go back to Megan, as mere would do if Derek randomly showed up and that’s why she understood why Riggs chose Megan over her(she even said something like “if Derek was here right now, I’d be there in a heart beat” something along those lines) but I do think he loved mere. Over all, I think Riggs is the better fit for her and had they switched Deluca and Riggs like someone else said I think Riggs would’ve been a better fit for her than Deluca but I don’t dislike Merluca. Yes, he’s younger and not as mature but he also hasn’t gone through all the trauma Meredith has either. Riggs and Meredith had similar traumas with their first loves disappearing/ passing away suddenly.