whats the best plant to fill the holes ib my conifer hedge

Do you mean to fill in the spaces between my conifer hedge?

Not a lot will grow beneath conifer hedges as I found out. You might be better to put a pathway (conifer trimmings?) between the hedge and your borders. Then you won't have to worry. You could also put a statue or ornament of some sort which will draw the eye away from the worst gaps. Alternatively, you will need to find a different shrub that might grow between. Difficult to know what might be best here.