I like a session beer like 4-5% not too hoppy, not too much alcohol, not too bitter, malty is okay but not too dark looking
For batter purposes I would stay away from:
Stouts, porters - too dark looking, great for braised red meats though
IPAs -too hoppy (actually if you are in a gastro pub setting with beer snobs who LOVE hops then this could work)
imperials - too high %ABV, too bitter
anything pumpkin (unless you want those flavors on your fish)
Belgians - too expensive for this use, also tripel, quads have too much alcohol
Did I rule out all the beers? Heck no! altbier, amber, english bitter, brown ale, steam beer, doppelbock, golden ale, hefeweizen, kolsch, light ales, saison, vienna lager, witbier should all work great in a beer batter.
Maybe go to smaller local breweries and see what they recommend. It's nice to get a collaboration on your menu and you say it's beer batter with such and such beer from ____ brewery down the street. Also you could do something like a beer pairing night with rotating local brewers It's a good draw for a slower night like tuesday-thursday and if you market it right you will sell out.