Altcoins that don't attempt to do what Bitcoin does "better" or "faster" appreciate in value alongside BTC. Really anything outside of a select few coins will at least maintain its BTC value (and if it holds value while Bitcoin appreciates, it's appreciating, too.)
Im not sure about that... we have seen huge ETH crashes, huge MAID crashes... as BTC climbed up. I think those aren't safe too, when a big ass BTC pump happens everyone sells their altcoins. At the end of the day, everyone just wants more and more BTC, they will sell anything they have to make their BTC holdings bigger. There are no safe coins when BTC pumps.
On a dip is a different story. But then again, people might sell the alts to buy the dip so....