In crypto world? Bitcoin. You do not get as guaranteed income as bitcoin, it doesn't drop as much as the other ones and it is also goes up very well as well, it is not going to make you 10x for sure, it is a coin at best gives you x3 and that is on the greats increase of the year, normally you are lucky if you can manage a 2x, now those look like great numbers but compared to some other coins that makes 10x it may look small but the difference is bitcoin also doesn't drop 80% all that easily whereas the other coins could drop like that in an instant.

So, if you want a "guaranteed" one you should go with bitcoin for sure, there is nothing as better as that one. If you really want to pick an altcoin, I would say ethereum looks like it will be good.