Two popular rope options are polypropylene and Spectra (as well as Spectra's close relative, Dyneema) in either 3/8" or 1/4" diameters. 50' and 75' are common lengths.
3/8" poly has about a 1900 lb breaking strength. 3/8" Spectra comes in at about 5000 lbs. 1/4" poly is about 950 lbs compared to about 2500 lbs for Spectra.
Poly has a melting point of about 330 degrees F and has poor abrasion resistance. Spectra is polyethylene fibers wrapped in a polypropylene sheath and has excellent abrasion resistance however it has a low melting point (between 220 and 225 degrees F) and doesn't hold knots well.
In my experience, poly works very well for rescuing swimmers however it should really never be used for any type of rigging beyond a 1:1 direct pull (the boyscout method).
3/8" line is much easier than 1/4" to hold onto in a resuce situation however 3/8" is obviously much bulkier.
If only looking to carry one rope, many folks opt to spend the extra money for 3/8" Spectra - just understand that the heat generated by friction in a haul system needs to be addressed.
I hope you find the information helpful.