You can't really expect a good long-term returns from an altcoin, altcoins just have pump and dump cycles, and they also just follow Bitcoin, but Bitcoin outperforms them. Still, while most coins are shitcoins, maybe a few could be undepriced right now. My choice would be Monero, it has been stagnating for a while, and its price in BTC is very low historically. Monero has decent fundamentals because it's the best privacycoin, but privacycoins aren't popular among investors because of the regulation risks.
That's certainly true, mate. But I have to admit that there are some coins that are unique, despite being so old in the crypto/Blockchain space. They've managed to remain actively traded on the market after all these years. Monero is a good example of an old coin that has stood the test of time. I remember when XMR was worth cents (in USD), only to be worth more than $50 a few years later. It turned out to be a good investment for me as I've managed to get in early. Besides price, XMR is very useful for privacy-oriented transactions. Everything is virtually untraceable giving you peace of mind over your hard-earned money. The only issue would be mainstream acceptance, since governments are talking negatively about privacy coins in general. Even now, some exchanges have de-listed Monero and several other privacy coins because of mandatory KYC/AML compliance. We might see further de-listings until privacy coins like Monero and Dash are used and traded "under the radar".
As far as old coins go, I think Dogecoin is one of the best performing ones on the market today. The community is still vibrant supporting it every day. The only issue is that there's no active development. But I often enjoy DOGE's low fees and extremely fast transaction confirmation times (compared to leading cryptocurrencies such as Bitcoin or Ethereum on the market). Other old coins haven't been doing so well, leading them to being de-listed from every single crypto exchange in the industry. But their Blockchain networks are still functional to this date.
Monero, Decred, Siacoin. Forget all the rest. Not that other coins will lose value. they will stay as valuable as are now but these 3 will gain a lot in the next decade.
Glad to see someone mentions Decred as one of the best performing old coins on the market today. It may not be in the top ranks in market cap, but it's still "kicking" after all these years. The fact that Decred has an innovative approach to PoS, makes it undervalued in my opinion. It's a hybrid PoW/PoS cryptocurrency which gives it extra protection against 51% attacks. This, alongside many other exciting things about DCR (like Stakepools), makes it a great long-term investment.
I'm personally "hodling" Monero ever since I got into it in early 2015. Prices have increased astronomically since then, as there's been a surge in demand for privacy coins. While now ETH and stablecoins are getting all of the attention, old coins like Monero and Decred are still relevant these days. I'd say it's best to diversify your investment into new and old coins in order to minimize risks as much as possible. Just my thoughts