Wow this requires some thought! At this moment in time I'd probably say the "BILY" scene in TTLG finale. It's the ultimate proof of Jack's unconditional love for Kate. Jack was willing to stick up for Sawyer and stand aside to let her be with him if that's what Kate wanted. In that moment he was putting her happiness above his own. And for Jack letting the woman he loves go is something we've seen him have a LOT of difficulty in being able to do - he pratically stalked Sarah, and even with Bai Ling he got way too attached and needed to know she wasn't a prostitue. However for Kate, Jack was prepared to finally do that and it's so amazing...shows how much he loves her, he'd do anything for her happiness...he's just a fool for not knowing that actually Kate wanted him all along not poor used up Soya bean. :P

I also think that putting aside his problems to help raise Aaron with Kate was wonderful, even if it didn't last long....

Yeah! I agree with you! The two you have mention were great romantic gestures! Two of my fav too!
I also love the one in s3 when he performed Ben's surgery because Kate asks him to(or they would kill Sawyer), even though he had saw her and Sawyer in the cages. Another one I liked was in s1 when Kate was supposed to carry the dynamite after the mission to the Black Rock but Jack secretly switches their backpacks in order to carry the dynamite himself to protect her! <3

Ah yes the backpacks!! I completely forgot about that! Yep that was wonderful and so early on in their relationship (lmao I started typing marrige!)
I also love what Jack said in TPIS this season when he said he'd kill Ben if Sun didn't because of what he did to Kate. That made my ovaries melt!