I think a couple of Alts are definitely worth holding.
Most of the alts are rip offs or similar coins then Bitcoin, sometimes even forks which can be okay of course but still they are all following he leader (BTC).
Although I know it sounds cliche, some alts are using completely different technology and are steering their own path with its own technology (of course inspired by Bitcoin).
Let's have a look at IOTA and the Tangle (for a clear and visual explanation
https://theblockchaintoday.com/iota-the-tangle/ ). The technology is very revolutionary as the speed increases over time and there is no need for miners and therefor no fees. This has a lot of potential, not even as a payment solution but a way of sending and storing data.
IOTA is one example that really brings something new to the table IMO.
Next to that we see others like NANO doing something with similar benefits.