Largest ones are Bitcoin and ETH. There are some other altcoins, but who can predict price movements, even in some months ahead. So if you invest in BTC or ETH, you can hope, that after three years, the price will go up.

I think you have to diversify your capital so that it will be safer and more profitable, you can choose bitcoin and Ethereum for long-term investments and other altcoins for trading, why buy this year? because this year prices have dropped very deep and it's a good opportunity to sell
It is not up to a specific year or season but it keeps on changing in different times for crypto coins like the Perfect time to buy or sell Bitcoin or any other coin then you will have to monitor current conditions for this purpose as things keep on changing in crypto market.
If you want to buy currently then you should proceed for Bitcoin as it is available on low price and you will get it easily but remember that it is a long term investment.