Preferably whole, though spears are okay. The dill-er the better for the proper sandwiches: eg roast beef, Rueben and such.

Sweet pickles (sliced) go great on hamburgers (at which point I use mustard and not ketchup). I don't know about appetizers, but I'll smack on pickles. Either the smaller sweet ones, or the big juicy Kosher Dills.

Watermelon pickles sound great!