A couple of the rods I fish frequently are Wright & McGill/Goodwin Granger bamboo rods from the mid-1940's. The guides on these rods are much smaller than the guides on modern rods, and the leader to line connection must be smooth and small to prevent hang up either coming in or going out through the guides.
I also fish exclusively with furled leaders that I make myself, and I always put a Shorb loop on the butt end of the leader.
The best connection I have come up with, and believe me, I've tried them all, is to first soak about an inch of the fly line tip in acetone for about a minute. Then take your finger nail and scrape the softened plastic off the fly line, leaving about an inch of core exposed.
Then chuck up the fly line in in a fly vise and form a loop of about a half inch in the core. Using a fly tying bobbin and 6/0 uni thread, make 20-30 turns of thread over the loop, taking care to build a "ramp" of thread from the loop back to where the fly line plastic coat starts. Whip finish and put a small dab of super glue on the thread wraps.
Then I loop to loop the fly line loop to the Shorb loop on the leader.
This system gives me by far the smoothest leader to line connection I have come across. Even with the leader three or four guides inside the rods when trying to net a nice fish, I've had fish make a last run and have never had the leader-line connection hang up on a guide.
Conversely, the absolute worst loop to loop connection I have tried is the loop that many manufactures are building into newer fly lines. That thing is way too bulky and hangs up on even my modern fly rod guides. I have no idea what possessed line manufactures to start doing that- are you listening Scientific Anglers?