As long as there have been humans, there have been rats living alongside us. Omnivores like ourselves, rats can eat just about anything we can and in fact will usually prefer whatever their person/people happen to be eating over any purchased pet food. Just try eating something next to your rat and see for yourself...
Rats like us, benefit from a balanced and healthy diet. But unfortunately, also like us, the food they want the most is not always the food that is best for them; in particularly rats love sugary and salty foods.
That said, one of the quickest and most effective ways to build trust and friendship with a new rat is through offering food so having desirable but healthy treats on hand is always important.
One other thing to factor in, especially if you are using the treats to encourage new rats to spend as much time with you as possible, or to train new tricks, is size and consumption time (ie how long does the treat take to eat?) You will find that rats tend to try and stash larger treats rather than eat them on the spot, firstly so that they can eat them somewhere they feel safe or later, when your rats trust you, more because they want to get as many treats as possible and don't want to waste anytime eating the treats whilst they could be adding to their stash.
When looking to build trust and friendship with a new rat, or training new tricks, I prefer treats that the rat can either eat in one mouthful, or better yet, that they actually lick off my fingers. Most treats can be broken down into tiny parts to achieve this goal so with no further adieu, here are my top treats:
Crunchy vegetables - eg. cucumber, carrot
Real Yoghurt (no sugar, no sweeteners) just milk and cultures
The crusts my kids won't eat
Porridge
Pasta or Rice (usually with the sauce)
Fish
Cooked meats - leftovers (only small amounts) - chicken, bolognese sauce etc
Blueberries and other fruits
Eggs
Healthy snacks I make for the kids (any leftovers)
Clover and Dandelion leaves (that's right, my garden weeds - just make sure they have not been sprayed with anything, the Rats love them)
Bugs (Rats will actually hunt anything smaller than themselves)