Out of all the 401K accounts provided by Fidelity Investments, which are the best to invest in, long-term?

I'm not sure I understand the question. As far as I know, there's only one type of 401(k) account. Within that account, you can choose many different things to invest the money in. If you mean which of the Fidelity mutual funds is best to invest the 401(k) money in, then the answer to that would depend on how long you have until retirement and whether you can avoid the temptation to move the money out of stocks when the market falls.

For highest returns over the long-term, historically that has been stocks, with small company stocks doing slightly better than large companies and "value" stocks doing slightly better than "growth" stocks.

What I'd do with my 401k money, assuming I'm not retiring for at least 8-10 years, is put about 40% in a small-company stock fund, 40% in large-company stock funds, and 20% in international stock funds. That's more aggressive than most financial planners would probably recommend, but I'm willing to ride out the periodic collapses of stock prices (like we're seeing now) to get higher returns over the long run.

When it comes to long-term investing, I would cast my lot with low costs. Fidelity has the lowest cost index funds available to small investors. Their "spartan" index funds have no loads, no 12b-1 fees, and an annual expense ratio of 0.1%. Can't beat that. Simple pick an asset allocation that matches your needs, using a mix of their total stock market index fund, total international stock index fund, and total bond market index fund.

See chapters 19 and 23 of my free downloadable book for more info on costs and asset allocation: http://www.invest-for-retirement.com

Depends on what is offered in your plan and your age, among other things. The easiest "hands-off" choice is usually the target retirement date funds. You basically need exposure to (1) domestic stocks, (2) foreign stocks and (3) bonds. The best mix depends on your investment timeline.