The question is right but the follow up frames it to the current big coins. You should reframe to seeking the next generation big coins. Coins that are going to solve current problems of current coins.

Personally, I think that:

• scalability
• fees

are the next things to be solved.

In terms of currencies (not smart contracts etc), I see 2 upcoming coins:

• IOTA (already quite known but beta stage)
• Railblocks (is only just coming out of the woods. Barely known, not vested yet but very devoted developer)

As always, each of these investments (including Bitcoin) run the risk of going to zero. Only invest what you can afford to lose completely.