I'm a bit confused by what you mean when you say "collage". If your just blending separate photo's into one single photo then I would say Photoshop is the best... and one of the most expensive as well.
For a free image manipulation program try GIMP. It's a heavy duty free ware program that is comparable in many ways to Photoshop. Learning all it's capabilities can be a chore but get the basics down and you can do wonders.
I desperately need to make a nice collage but only one. Which photo program is the best for this? I have Picasa2 already but used Irfanview in the past, but simply don't know which one would work the best.