I would say that by far the number one most useful set of plugins is
the Matrix1Utils plugin collection, by IanM. It adds several hundred new commands to DBP, many of which should probably have been in DBP from the start.
Just a small look at what it provides:
-Function pointers.
-Large set of string functions.
-Flood fill.
-Find Free Object()/Sprite()/Sound(), etc.
-Console output window.
-Large set of array functions including array pointers.
And that's just the tip of the iceberg. These plugins have saved me countless hours that would have been spent writing functions that should already be in DBP.