I don't think that Bitcoin will die anytime soon, but in the (very unlikely) event that it loses adoption and does die, I think that LTC would be a good contender to become the biggest cryptocurrency - it has risen pretty well over the last year or two, to only have a small(ish) drop recently. Another contender would be DOGE - it is holding its value pretty well at the moment (even though each coin isn't worth much), and there is no limit to the amount of coins produced, which could help it do well (not sure why, just a weird theory).