Normally a looped fly line but I've also used Eagle Claw leader links. They work well. No cutting back fly line. Simply add a new leader when required.
I use the slip on leaders loops too, but I like to grease it up real well before I begin fishing and periodically thru the day. Otherwise they tend to soak up water and sink the tip of your line.
I tried the slip on loops but did not care for them. On lines without a factory loop, I use a length of mono nail knotted to the fly line and tied with a surgeon's loop for a loop to loop connection.
I use a mono loop connector. Nail knotted to fly line with a perfection loop on the mono. They are very durable but I usually replace with a new one at the beginning of the season mainly because it just makes me feel better. I have never had one fail on me yet.
I also use a mono loop nail knotted to my fly line. It's a pain to tie properly, but makes changing leaders really simple once it's done. I use a surgeon's loop usually - I've had a perfection loop come undone once... Probably my fault, but I don't trust them anymore.
Man, I guess I'm ole school. I find something I like and it's hard for me to change. Maybe I should try some of these other things. I still go with a pliobonded Nail knot for the Butt to Fly Line connection and blood knots. I cut the loops off leaders if I use a factory knotless and blood knot the leader to the butt section. I don't mind tying blood knots.
Nail knot. I cut the factory loop off the line too...With a nail knot tool I might only clip a half inch of line each time I change the leader. My butt section of leader gets changed maybe once a year. That's a half inch a year of line lost. I like a sleek transition from line through leader.