Who knows... could be Bitcoin. Could be Litecoin. Could be Dash. I'm sure there are a few other potentials. Oh yeah... could be Gulden ;)

Bitcoin will remain relevant for sure up until all coins are mined. People holding Bitcoin want an need that to happen. Price will surge for a while at that point. Then people will get rich and convert to fiat. Bitcoin will not be the coin to massively used I think though just because of the transaction speed. It's too slow. Who know how much they lower that time...

Looking at transaction speed, litecoin and dash win.