The classic lemon and sugar is a good'un, for sure.
But – speaking as a guy that makes and eats pancakes about fifty times a year (for the last seven years or more) the absolute Rolls Royce sweet pancake filling is lightly sweetened cooked fresh raspberries, seived, with a little double cream.
Don't forget a pinch of salt in the batter. I also put a pinch of cinnamon and a grating of nutmeg in the batter – not enough to dominate, just sufficient "haunt" the aftertaste.