It will not be Stellar. It is centralized. Ripple is more popular than Stellar.
I think Ethereum will be the biggest altcoin.
Ethereum is not an altcoin and neither is ripple. Therefore eth/xrp do not qualify as a bitcoin replacements.
Ethereum is a cryptocurrency.
It's also the cryptocurrency that has the highest chance of being bigger than bitcoin.
I'm holding a lot of Ethereum but I don't believe it will likely replace or become bigger than bitcoin period.
I did also hold a few k ETH but they recently got sold. There is no coin out at the moment that is going to become bigger than BTC especially ETH. If you brought when you created this thread OP then you could have had some nice profits but I would not hold thinking it could replace BTC because in my mind it couldn't.
If anything was to happen to BTC i think the coin to take of will unfortunately be DASH but I would prefer it to be XMR.