Thaumcraft and Thermal Expansion, basically. To me, those two mods set the foundation for modded minecraft. Although, I would probably end up bored quickly if it was only those two in my pack. Neither of them are perfect though. Thaumcraft can be incredibly tedious without other mods to speed things along, and the whole research/warp features are like punishment which is why I disable them in the config. And Thermal Expansion on its own hardly does anything at all. But both of them have excellent gadgets and the crafting recipes are usually pretty fun to follow as long as you arent doing infusion altar recipes one after another.
A close second in the tech mod department would be Mekanism. Its got a lot of easy to make, fun to use machines. But almost everything in the mod seems slightly...off. In the way of the old universal electricity mods never quite meshed with everything else around them. The textures are by default, 32x while default minecraft is 16x. And as much as I like playing around with fluids and item pipes, the addition of gasses seems like overkill to me.