Sublime Stitching Fine Tip iron-on transfer pens.
Black and blue are good for any fabric (except dark obviously!)
Orange and red - these colours are really useful if you are stitching something delicate. They only make a faint line so it's important to only use them on LIGHT coloured fabric.
They are a really good choice for when you don't want a thick line of colour (like the Sulky Transfer pens which can also get a bit blobby).
You can get a couple of impressions from one tracing. Works really well on plain cotton, less so on linen that has a bit of texture to it. They still work but if you want a really strong line to follow, go over it with a water erasable pen.
Make sure you warm up your fabric first. The transfer is permanent so DOES NOT wash out - you will need to stitch over your pattern to cover it up.