I don't think that there is a 100% accurate answer or definition for the best altcoin or cryptocurrency in general. It depends on the needs of the user, the budget, the preferences and, of course, the potential that the cryptocurrency has for further growth.
For example, some users like ETH, because it has the smart contracts, which give the software developers the ability to design algorithms that may serve their needs the way they agree upon.
Some users like LTC and think it's the best, because it's pretty much like BTC, but is much more cheaper and easier to acquire and store (and transfer).
Some users like XMR, because they want their online activities to be tracked at all, and XMR has the full potential for that!
Some other users may prefer XRP, considering its potential of making global transactions much more faster and cheaper.
To me, for example, all of the features mentioned above are very important, so I like all of the those coins because of their different abilities and characteristics. So it would be hard for me stay which of them is the best, because they are all equally great in their respective fields!
