On each level, it is dependent on the situation. If you can't get up really steep inclines, it's a good idea to upgrade your engine so you can climb the harder slopes. It's also worth noting that replacing your tires will improve your traction.
Suspension and 4WD enhancements will also improve your handling significantly. If you're not having any problems, we recommend replacing all four engine systems at the same time, rather than focusing on one in particular.