The green swim stim koi pellets from Dynamite are a definite winner in my book.
They are very light, so I tend to moisten them with wet hemp and this is a pretty deadly, if expensive, combo for me.
If I use any groundbait in winter, then it tends to be VDE Secret, mixed with Grilled hemp and a very small amount of brown crumb.
I keep it light and fluffy and only feed very small amounts.
In winter, don't forget to add a teaspoon of salt to all your baits.