Best line for vertical jigging ???

I usually use 6lb mono

I use all mono or copolymer.  A good rod lets you feel when the fish hits it so that is not an issue or good argument for me to use no stretch.   it's pricey,  it's pain in the rear at times.  I get a snag I bust it off.  jigs cost me .12 cents a piece.  big deal. I've used no stretch in the past.  if you get snagged you get the jig back most times but you can only bend a hook so many times and it's shot.  vertical jigging boat control is more important than line anyway.

I run 6lb most times.  I have one with 8lb just in case the current is fast and have to use a 3/8 oz or heavier.  the mississippi can be a fast SOB at times.

I use no stretch for lake trout fishing in 120 feet of water,  that's the only time i feel it's warranted.  Or casting cranks...sure.  $6-8 baits,  it makes sense.

I love nano for jigging with a flora

Leader nano is expensive and has to be replaced often but it is extremely sensitive

Depends on where you are jigging . In a clear lake I would agree with your 6lb mono . In a snaggy river I would run 10lb hi vis fireline . I take a black marker and color the last 2 feet of line . The fireline has no stretch so I can feel exactly what the jig is doing . I can feel the bottom and see if it's soft or hard and if I get hung up on a stump or rock I can usually get my jig back . In a river the fish are usually behind some kind of obstruction so you usually have to bounce the jig over it and into the fishes face . No stretch lets you feel right when the fish hits it . WHAM! FISH ON !