Strong sweet-and-sour sauces are some of the best accompaniments you can get, but you need to get it right. If the base ingredient is good enough (good quality meat or fish, properly strong cheese) then it's best to leave well alone, but to enliven substandard ingredients (and there are far too many of those):
Ketchup is just about OK on fish-and-chips in the absence of tartare sauce.
For breakfast it has to be brown sauce unless there really isn't any alternative.
With cheese or in sandwiches (meat or cheese) I wouldn't touch anything apart from chutney or pickle, unless it's cheese-on-toast, when worcester sauce and/or raisins are called for.
Lamb requires mint sauce or chilli sauce.
Seafood in general wants a hit of heat, whether that's chilli or chorizo.
For salads, I prefer my sweetness to come from pine nuts or raisins. (Which reminds me - one of Pizza Express's best inventions is the Veneziana - capers, onions, pine nuts and raisins).
Mustard and horseradish are repulsive inventions, fit only for use as emetics.