Which one is a better investment, Bitcoin, Ethereum, Litecoin, or Ripple?

I think that in the long run investing in bitcoin provides higher reward/risk ratio. At least for now.

Ethereum is very promising but very risky. It is still not clear that real life decentralized applications built on Ethereum can work and scale. The working dapp that I know about - the DAO - ended as a disaster. Not only the smart contract had bugs that were not spotted by developers, but the investors were able to pressure Vitalik Buterin - the ethereum founder - into providing a bailout, i.e. reverting the contract and giving a new meaning for “smart contract” as it is more “smart” than contract. Contracts are not supposed to be broken just because one party just signed up based on promises instead of reading the contract. Is it possible to write smart contracts without bugs? When those contracts with bugs get canceled? What will happen the next time people will lose money because of bugs? Will they able to pressure Vitalik? And if so - what is the value of ethereum if the contracts are reversible?

Another issue is - Ethereum is supposed to switch its Proof Of Work mining algorithm into Proof Of Stake. Protocol change is not trivial in living working system. Will it be done flawlessly?

And the last but IMO most important factor - there’s currently no cap on the amount of Ethereum tokens, so the inflation rate is unknown is it is possible that the number of Ethereum tokens is unbound which certainly means the price per unit will have constant downward pressure.

Regarding Litecoin - IMO in the long run it had little potential. It is very similar to Bitcoin and will probably just lose market share eventually. Off course it might benefit in case bitcoin will fail to proceed with a solution to the governance problem, but who said Litecoin would not succumb to it too if it wins bitcoin?

Bitcoin is the safest and has the best developers, community, and benefits from the network effect. It has issues - but those are not unique to Bitcoin - it just encountered these first.

And in the short run - everything is possible.