As Precious Tijesunimi mentioned, it is wrong to flush a fish, live or dead (perhaps especially not live, as this is how many harmful species get introduced to environments they don't belong to... not to mention it is cruel). Especially don't place it into a "natural" ecosystem! In fact, both of these options may be illegal in many areas.
Depending on your area, cremation may not be a legal option (I know some areas have restrictions on open fires of any type).
Burial and trash strike me as the best solution. However, there are some caveats:
If you bury it, bury it deep enough that neighborhood critters (cats, raccoons, dogs, or any other scavengers) don't smell it (you don't want to smell it, either!). Consider placing a decent-sized rock over the burial location, both to discourage digging, and to mark the spot as a memorial.
If you throw it in the trash... place it in a ziplock bag, first! Rotting fish is a very pungent odor, and your neighbors and your trash collectors will appreciate this basic step towards preventing the stench from spreading. Plus, it will also reduce the chances of critters trying to go through your garbage until it is collected.