I agree with Woodduck. I would add Massenet and Tchaikovsky as those who wrote masterful overtures that match the overall quality of their operas (Prokofiev too I'm inclining to include). As far as great or effective overtures, but not so great operas are concerned, some people (or critics) would point out Lalo's "Le Roi d'Ys", although I find the opera quite underrated. Magnard's "Guercœur" has a very arresting overture, although the opera itself is banal and meandering in places.