Tough questions, as we can't really tell exactly.
Here are few altcoins I invested in:

1. Tradove B2Bcoin - The first B2B token in cryptomarket, with a great marketing and huge community, though the price is falling significantly since ICO, in my opinion for long-term this is a really great project.
2. NANO - I guess you know why I picked this.
3. Ethereum - Really? Do I need to explain this anymore?

There are a few more actually, but these 3 are the best altcoins to invest in my opinion. Of course, it varies for one another, just DYOR and pick it yourself.