Can humans be considered carbon sinks? If so,for how long? What living organisms are better long-term sinks?

The question of which is better has less to do with the organism and more to do with how it dies. A tree that burns to the ground will release all of its CO2 instantly, but one that gets compacted into the Earth and over millions of years turns into coal is quite a good one.

So if you'd like to help out a bit with CO2 levels, opt for a coffin instead of cremation.

Yes...

We're just food for the plants and trees.

Bacteria would be the most long term carbon sink...