The best time was long time ago, lol. But nobody didnt knew about bitcoin and ethereum those days, so invest today and maybe we will say the same again soon.
The best time to invest in ETH, I think, is probably the last one, and it is possible to assume that eth has surpassed the most in 2017, where there are many billionaires and ETH millionaires. With the market price not rising but decreasing, I think it is not the time to invest in ETH. Similarly, not only ETH decrease, but also some digital currencies such as bitcoin, altcoin, bitcoin is significantly reduced compared to last year reached more than 400 million Vietnam dong. If you do intend to invest in eth I advise you to break in to a later time or you can move on to explore other developmental currencies.