if you're only putting your money in a single ETF/MF for an extended period (e.g. for retirement), it should be incredibly diversified. Target-date funds from an efficient provider (Vanguard) are probably your best bet if that is your goal.
If you want to be diversified and you're not married to the idea of one fund to rule them all, then I recommend buying a basket of the following ETFs (with % allocations adjusted for your risk appetite):
Stocks- VTI (Total US Stock market)
- VEA or IEFA (Developed Market stocks, excluding US, i.e. foreign, established large markets)
- VWO (Emerging Markets stocks, i.e. foreign, more volatile
Bonds- BND or AGG (Mostly US Gvt some US IG corp bonds) maybe MUB (Municipal Bonds) too if taxable
- BNDX (International Bonds)
- VWOB (EM gvt bonds)