Personally I like the C fund it follows the S&P point for point. If the S&P goes up 1% then so does the C. Real easy to track your earnings over time.
Since I'm one of the Dave Ramsey guys on here, Dave recommends 60% C, 20% S and 20% I - I follow that and it has done all right by me.
The big key, though, is to make sure you get the full match. A positive feature that I don't make nearly enough use of is - there are no explicit transaction costs, - the cost/expenses, etc., of the fund come out of your yield - you can trade into or out of the funds every day if you want, at no additional expense. The trades are not in real time, so you basically get market prices on the buy, but they're all index funds (or bond funds) so intraday timing isn't a huge deal, anyway.