Whenever you mention Altcoin, my mind first goes to Ethereum, if the Crypto boom started right now, I would be among the first people who would buy Ethereum, atleast add to the ones I already have, I love Ethereum because it's one of the easiest blockchains to use and I am conscious of the contributions that Ethereum has done to the development of blockchain technology, another coin I would consider purchasing is XLM just out of pure sentiments because the XLM airdrop was one of the best Airdrops I ever participated in.
Investing in ethereum is one of the best choices, it is the most popular altcoin in the market because of its uses.
This is also the one I always think when I think of investment, instead of investing in any other altcoin, I choose to invest in ethereum and nothing else.
And why?
I'd probably stick with the tokens I am holding right now in order to avoid mistakes on choosing which other alts would be profitable investing if ever the price starts to rise. It is a way to be safe from regrets since if ever there will be a price increase, we don't know how long will it last and if it would positively affect ALL of your holdings. But it would depend on how confident you are as an investor especially if you really trust such coins of your choice.
I do the same thing, investing in other altcoins aside from the one I am holding is much better than trying any other altcoins. It is way better to increase the number of a certain altcoin I am currently holding.