I haven't read all the posts, but did vote ketchup. Brown sauce is just a bit nuts, I've used it a few times and it's just not for me.
However! I must throw into the ring my preferred option, and if you haven't tried it with your preferred meat or meat-alternative then I'd highly recommend it:
BBQ sauce
In my case, specifically Sainsburys' sweet and tangy BBQ sauce. That one is vegan-friendly, hence my specific choice. Yesterday, I had a Warburtons soft pitta bread with 3 Linda McCartney Lincolnshire-style sausages, adding a bit of BBQ sauce to finish the bottle off, and wow that was nice
There's plenty of things I'll have BBQ sauce on, and that includes microwaveable rice (try it, go on I dare you, it's amazing) or if I'm feeling ultra lazy then I'll just put a load of it on slices of bread and put them together. I've also used it with a plate of vegetables before now, again I dare you to try it! Go on, try it and when you love it thank me later
Fully expecting some reactions there, of course!