It is hard to say where to invest these days, since nothing is a sure bet. Historically, people have invested in plex. However, plex prices are very high and the CSM was very vocal with CCP to the effect that they think plex is too high - and CCP responded by noting that they have various means to allow them to lower said price.
What is it that you do in eve that you enjoy? My suggesting would be to “invest” in what you like doing. From there, keep an eye out for potential isk making opportunities (although I suspect you already do this).
Alternatively, you could also invest in a trustworthy “fund”. Here’s a link to my fund. Investing 100bil would give you 2 bil a month guaranteed; 200b would give you 4bil a month guaranteed. Other funds are also available, but do your research first as some appear to be scams.
In any event, congrats on the accumulation of wealth and best of luck to you!