1. Total Stock Market
If you don't mind the volatity, you could do a lot worse than picking a single fund that covers the whole US stock market. It is simple, very low cost, diversified among 3,000 different companies, and has a long track record of exceptional returns.
Vanguard
Vanguard Total Stock Market
VTSAX
100% Fidelity
Fidelity Total Market
FSKAX
100% Schwab
Schwab Total Stock Market
SWTSX
100% 2. Balanced Index
Fancy using just one low cost fund? This fund gives you all the stocks in the US and all the bonds in the US in a fixed 60/40 balance
Vanguard
Vanguard Balanced Index
VBAIX
100% 3. Wellington Fund
One of Vanguard's oldest actively managed funds gives you a 65/35 stocks/bonds ratio. This is the actively managed alternative to Balanced Index, though it mostly behaves like an index fund, and is still low cost by fund standards and has shown consistently good performance over decades.
Vanguard
Vanguard Wellington
VWELX
100% 4. Two Fund
The two fund portfolio is a simple mix between a US stock market and bond funds, as recommended by the likes of Jack Bogle and Warren Buffet. The classic portfolio is 60/40 stocks/bonds, which some might find too conservative, so simply adjust for your risk tolerance.
Vanguard
Vanguard Total Stock Market
VTSAX
60% Vanguard Total Bond
VBTLX
40% Fidelity
Fidelity Total Market
FSKAX
60% Fidelity U.S. Bond Index
FXNAX
40% Schwab
Schwab Total US Market
SWTSX
60% Schwab Total Bond
SWAGX
40% 5. Target Retirement 2040
The default in many retirement plans. The fund uses all US and international stocks and bonds in an asset allocation that gradually gets less aggressive as the years go by. Too international heavy for my taste, but nonetheless a solid fund.
Vanguard
Vanguard Target Retirement 2040
VFORX
100% 6. The Polite Investor
I use a very simple two fund porfolio that I tilt towards growth and away from international. I wanted something similar to the Wellington Fund, but with more control over the allocation.
Vanguard
Total Stock Market ETF
VTI
55% S&P 500 ETF
VOO
25% Inter-Term Bond ETF
BIV
10% Inter-Term Corp Bond ETF
VCIT
10%