In non-written rule is that tne flagbearer is someone tha has a previous gold medal. Taking that, it's between 3 people:
Saul Craviotto (Canoeing): Olympic champion, multiple medallist and pretty popular and well known face (He's a big star since he won Celebrity Masterchef, nation's biggest tv show nowadays). He was already the flagbearer at the 2012 closing ceremony.
Mireia Belmonte (Swimming): Olympic champion, multiple medallist, well known face, 2018 Mediterranean Games flagbearer . Only thing against her is usually swimmers are not alowed in the opening ceremony since they compite next day...
Lidia Valentín (Weighlifting): Olympic champion, multiple medallist, known face (not so much compared to the other two), In her favor is the "social justice" of her having three medals but only one podium in her oympic carreer due to dopers, so she deserves an olympic big moment.