i.e. Currently to add a filter you to the bottom, clicking filter, selecting a column name, selecting equals, entering a value.

Instead, right clicking on a cell and selecting "filter > equals" would apply a filter to that column for values matching the selected cell's value. Likewise "filter > contains", "filter > does not equal", "filter > greater than or equal", etc.

These filters would then be appended to the filter at the bottom, so could still be managed there; but just saves some effort when first populating.