Bitcoin:
is decentralized, it has a lot of adoption, it has a lot of users, a lot of usage, a lot of real world applications and a lot of services that you can pay for with bitcoin. the distribution is the best, there has never been any premine or any bullshit to get the coins, you could get them from day 1 if you wanted and there is still no restriction, you can buy or if you are willing you can invest and mine it.
the market is the biggest and that means speculation on it is a lot easier than any smaller markets.
there is real adoption where new people come and buy bitcoin because they either want an excellent investment or they want to use this cryptocurrency online as it is intended to (a peer to peer decentralized digital cash).
and a lot more.

Dash:
it has a decent amount of supply and that can help its inflation and finally the price. it is a pump coin and that is a good thing for making a big and quick profit.
it has a big premine. and many even call it a scam. it doesn't have a positive public face. there really isn't any real world usage for it.
in the anonymity department it is not really anonymous. and compared to other coins it has nothing to say compared to coins like Monero.
there is no adoption, only traders switching their money over to Dash and out of it after they are done.
it is a pump coin and that is bad because it kills the coin in the long term when it is pump and dumped.

ETH:
it had a good idea at first but it is implemented pretty badly.
the code is bad and has many bugs, many of which has already been found. some of them even exploited and one of them led to a hard fork and splitting of ETH/ETC and there will always be a possibility of the same exploits and other unknown bugs that can be devastating to anybody who has his money in this coin.
it has a big premine (in form of ICO).
there is no real world usage for it and the smart contract that they are so proud of is not really something special on this platform.
the supply is HUGE. it has one of the biggest number of coins available and with a simple supply and demand rule the price will always be under constant sell pressure.
there has not been a clear cap on the maximum number of coins available.
the last and the worst but not the least is the centralization. this coin is one of the coins that is completely centralized and everything from the price to the code and the future is being decided by those who control it and can easily be manipulated to their advantage.
it is a pump coin so that is a good point of this coin and that makes it a good investment as long as there is a big pump going on and we can get in early on.