The best line by far for tarp ridgelines is polyester covered dyneema or arborists dyneema braided throw line. These lines have zero stretch, which means you tarp will feel much more secure and quieter in the wind.
These lines also don't soak up water so your kit stays drier and lighter. Its also much less prone to tangling than standard poly or nylon cords. Well worth the investement if you plan to hang your tarp regularly.
Google either Kingfisher dyneema rope, Marlow racing dyneema, English braids dyneema for my favourite lines at the best price. These types tangle less than any other. 3-4mm dia is what you want.
If you want really light, go for a spool of New England ropes dynaglide, Stein 1.8mm throwline or samson Zing-it.
If you want highly visible/reflective lines at night, go for Expeds line or another reflective guyline-I don't like them though, they are too bright at night for me.
For cheapness, my favourite paracord is the cheaper polypropylene stuff-its much lighter and soaks up much less water than kernmantle paracord.