I think you have already passed the best investment opportunity as for late 2018. In December the prices were extremely low, but as for now the prices are still great. If you forget ETH was around 1400 a year ago, so I think it is still great.
you're right. At the end of 2018, early in 2019, altcoin prices were very low, which was the best time to invest and easily make a big profit. But now the market is recovering very quickly this is also a good time to enter the market as it is possible that altcoin prices may rise further later this year.