Is not something I have ever been asked, but should it be on the tip of your tongue, here is the answer:
1 – Aceria macrochelus (155 records). A mite producing largish pimple galls on the upperside of Field Maple leaves.
2 – Aceria aceriscampestris (151 records). A mite producing small pimple galls on the upperside of Field Maple leaves.
3 – Neuroterus quercusbaccarum (57 records). A gall wasp producing common spangle galls on Oak leaves.
4 – Pontania proxima (54 records). A sawfly producing bean shaped galls on Crack Willow and White Willow leaves.
5 – Hartigiola annulipes (37 records). A fly producing “lighthouse” galls on Beech leaves.
6 – Aceria cephaloneus (36 records). A mite producing pimple galls on Sycamore leaves.
7 – Aceria eriobius (33 records) . A mite producing an erineum (a felt-like gall) on the leaves of Field Maple.
8 – Aceria pseudoplatani (28 records). A mite producing an erineum on the leaves of Sycamore.
9 – Phyllocoptes goniothorax (27 records). A mite producing narrow rolls on the edges of Hawthorn leaves.
10 – Iteomyia capreae (25 records). A fly producing pimple galls on the leaves of Sallows.
Maps of the 10 species, in alphabetical rather than rank order, are in this XPS file, which should open in most modern versions of Windows. The maps show records at 10km resolution for the two Glos. vice-counties.
https://www.dropbox.com/s/aw7qrw85bam8r31/galls%20top%20ten.xps
Robert Homan, Plant Gall Recorder