A smooth reflector is best for CFL bulbs. Try sheet aluminum. It holds its shape and if you find a polished sheet, it's around 85-95% reflective.
Your even cheaper option is taking any kind of general purpose glue, like Elmer's, or stick glue... cover a piece of cardboard or wood or plastic with glue(flat and thin but rigid is probably best) and then glue the dull side to the surface. If you have a really flat, smooth surface, take the aluminum foil and use a soft clothe and polish the surface first on that, then carefully put it on the glue-covered surface to retain as much smoothness as possible. Try not to get glue onto the shiny side of the foil(I've used an Ace Bandage, you can roll back any part that gets glue on it and keep going!). Start from the middle and use the clothe or whatever to get it as crinkle-free as possible. Your fingers can leave smears from oil/dirt and isn't nearly as even. Not only does it reflect great but it nearly absolutely blocks all light(especially the visible kind). Get foil tape to close any cracks. Or just put foil and tape it on with duct tape, or heck, glue it!
As for *best*? Probably silver. Over 99% across most of the spectrum when polished. Gold is *slightly* even better(but the price, ouch!). Then there's mylar which is up to 98%. People tend to distort *white paint* numbers. Unless you're powder-coating, you'll see max of 70-80%. Other (expensive) additives can increase this.
I've seen titanium dioxide/magnesium fluoride charts that claim over slightly over 100% reflection, due to the photo-active materials 'glowing'. Not sure if it means much for the PAR spectrum. Lumens alone aren't really a great indicator of how well a plant will grow under a bulb. PAR is photosynthetic active radiation. Classically defined as light waves between 400nm to 700nm. With the peaks being on the ends of this range. HPS is typically most intense around 570nm.
CFLs can generate nearly perfect black bodies, or CRI which is how closely a bulb matches sun light(as far as colors of objects are concerned). CRI isn't very important either. But it's not bad to have(fuller spectrum). GE Daylight bulbs seem very well suited to plant life. They spike around 400nm - 450nm and 650nm(great for plants). And otherwise have a smooth black body. CFL bulb manufacturers matter. They use expensive phosphors to tone light and generic brands might not fair as well.
What's most important is high bulb PAR rating and quality reflectors. A mirror-finished/polished aluminum sheet will be better than foil, and getting an Al alloy reflector would be even better. Mylar would be even better if you can find it cheap. With gold/silver being the best... if price is not an issue.
Proper reflectors can increase yields up to 30-40%. Don't mess with foil unless you don't have a better option. If you can get polished sheet aluminum good, 'professionally' cut for you, even better. Mylar.. great, might cost more.
The thing about aluminum vs white paint is that aluminum preserves the light spectrum much better than paint which cuts off intensity and is irregular. You would have to get a specialty paint and airbrush very smoothly to get near to aluminum. This is sometimes seen in reflectors. I've always heard no paint is better as far as most reflectors/bulbs. Paint causes more diffusion. This might be better for high watt HID. CFL's? No.
