I do career mode like 95% of the time. Mostly modded
Sandbox mode is fun, sure, but there's no real goals for me to go for. Because of this, I get bored with it quickly.
Science mode has the same problem, even if it aint as bad there.
Career tho. Contracts and managing funds (Even if I sort of cheat and go for 1.5-2x funds from missions) adds alot for me. And if you add some mods (Life support, among others), there's more of a challenge aswell, since you really dont want to lose your precious leveled up scientists and so on