Technically, the only way you're going to have a 100% Indica is if you're talking about a pure landrace strain from the subcontinent, or some cross between them.
Examples of strains like this include Maple Leaf Indica, Mazar, Afghan #1, Hindu/Afghan Kush, etc.
All the others are hybrids.
So with that caveat, if I had to pick only one it would be "William's Wonder".
Its not clear to me if this one is 100% indica; its probably been partially hybridized with a sativa for better outdoor grown in North America.
Regardless, its definitely indica dominant, and it packs a whallop like a sledgehammer.
Incidentally, nobody really knows the definitive genetic makeup of Northern lights #5. Some think its an indica-dominant cross between an Afghani strain and a Thai sativa. But whatever it is, its probably NOT a pure indica.
"Big Bud" is also almost certainly NOT a pure indica, either. That one has Skunk genetics in it, and the original Skunk supposedly contains both Columbian and Mexican sativa genetics.
So if neither Northern Lights #5 nor Big Bud are pure indicas, then a hybrid of two probably isn't a pure indica either.
It still may be kick-ass in its own right, though. It also probably *is* "mostly" indica.