Primochill with their Advanced LRT are the best soft tubing out there right now but if you're asking around for hard lined tubing you have a plethora of options. There are monsoon, EKHD, Alphacool HT, Primochill and ofc you can also pick them up from mcmasters. Soft tubing always discharge plasticizers into the loop and cloud the res/clear tubing and often gunk up in the blocks cooling fins.
Maybe if you could state the fittings your're planing on using would help narrow the field on ID and OD required/recommended...?
Oh and I forgot, welcome to the wild world of watercooling and to Tom's Hardware
