We Found Love>Born This Way>>>>>>>>>>>>>>>Firework
Edit: The above are my preferences. Now that I've read the OP, Born This Wat will probably be the most remembered. I don't use this word often, but it is "iconic" and will be remembered as a defining moment for Gaga and pop culture, whether good or bad. The other two will be remembered too, but less so.
Born This Way. It was the 1000th Hot 100 #1 and it set an iTunes record at the moment it was released. It's also a prominent gay anthem song (even though it's not its original intent).
Firework will be remembered to some extent, definitely more than We Found Love.
"Firework" by far. I don't know why "We Found Love" is winning. It's probably the best song of all three, but I don't think it will be that remembered as what people make out.