My best advice for this is "it depends!" Haha.
If you're not showing cleavage, I suggest filling them with a lighter material. They'll be less likely to move around and easier to wear for extended periods of time without being too heavy. I've seen some decent looks from people filling bras with foam (think couch cushion), etc.
There are some silicone breast forms on Amazon that aren't horribly expensive (<$50). I prefer these. They fill out a majority of the bra with a realistic size-to-weight ratio, then you can just pull your natural chest together toward the center to create realistic cleavage. Chest contouring can help accentuate that as well!
PS: Something that doesn't get mentioned often is how helpful a corset can be. In addition to creating the hourglass look, it also pushes your chest fat (if you have any) upward.
Some helpful YouTube vids:
Cleavage Contouring: https://www.youtube.com/watch?v=oca-4BXJCp4
Creating Cleavage with "Boy Boobs": https://www.youtube.com/watch?v=yjaKlp4Or5g