Some "light" 2D/3D engine, with a particle fx editor (so it can be easily imported into any project, 2D and 3D), a GUI editor (so I can visually make menus and the such), an object dropper, suport for multiple 3D listeners, being able to play more than 1 music at once, all streamed from disk... the list can go on and on.
The best thing is: "anything that can make game development faster, and easier, without loosing performance".
The best thing is: "anything that can make game development faster, and easier, without loosing performance".