Even though he was the character that was in the show least I voted for Doyle.
Freds Death was sad and thats my second choice, mostly because it was unexpected that she died.
Next would be Cordeilla, it's just that into season 5 I just kinda forgot about her and excepted she died back in season 3.
Lastly it would be wesley since it's right at the very end and I didnt feel that sad since its the last ep
Doyle however had an unexpected death, and he died a hero, he saved all those lifes willingly.