Really, there's no such thing as "optimal" because no one has a crystal ball. A portfolio of 100% VTI is indeed diverse, tax-efficient, and has low expenses. Personally, I'd add some amount of VXUS (international stocks) for further diversification and MUB (tax-free municipal bonds) for safety. You could do very well with just these three ETFs, often known as a "lazy" portfolio:
Keep in mind, though, that there are no guarantees. Stocks will have bad years, and you should expect that some years they will lose 50% of their value. The key is to choose an allocation that you can live with so you don't panic and sell when things get bad. Keep contributing and rebalance regularly (once or twice a year is usually enough). Tune out all the noise and stick to your plan. Even with these dips, most people expect stocks to perform better than any other asset class in the long term (20+ years).
I'd also recommend The Little Book of Common Sense Investing:
It's written by John Bogle, who founded Vanguard to bring diversified, low-cost investing to the masses. Vanguard is different from other companies in that it's client-owned. The Bogleheads forum is pretty good for this kind of investment advice:
You might also check out Wealthfront and Betterment. They are software-based financial advisors that use Modern Portfolio Theory and the Black-Litterman model to allocate your money optimally given their assumptions about expected returns and correlations between asset classes:
One reason is that you get greater returns by diversifying. Unfortunately, diversifying correctly is hard. A second reason is that younger people typically can afford to invest more aggressively and older people should typically invest more conservatively. So an investment strategy depends on your personal goals and attitude towards risk. Finally, it doesn't take personal aptitude into account. By investing in an index fund you can inexpensively get average performance, but you can get greater gains by investing your money in a way where you can do better than average. Start a company, real estate, art, etc.
