If I can avoid forcing them I do.
So far the things I've found that most of my rats want are
-Fruit Medley baby food by Gerber (almost never rejected)
-Nutrical (fatty, but good for old sickies)
-GREEK yogurt. Damned if I know why they prefer the greek yogurt, but mine really do. Yogurt should be a last ditch effort, because it can lower the effectiveness of some antibiotics.
For absolutely last ditch (like when I have rats with respiratory so bad that force medding them gives them horrible attacks) a bit of agave or whipped cream may push any of the above over into nummy, or a mix of butter and sugar. You mix the butter and sugar until when you try it you just want to eat spoonfulls of it forever, it should be very tasty. I've even done butter, sugar and whipped cream
For not so ill, but very stubborn, Lilspaz's method is very effective! Also works for nail clipping, though I've found that if you trap one hand while you do the other, they sometimes forget how to struggle