The answer depends on what kind of rewards your friend would find most valuable, and that is both an objective and subjective determination. For example, I realize that rebate/cash back cards often offer good value (measured objectively) but the fact is that I don't notice/appreciate small, regular rebates/cash back (so, subjectively, such cards don't offer good value for me). I prefer cards that provide travel rewards that I redeem for premium experiences (business class travel, luxury hotels) that I wouldn't otherwise pay for with cold hard cash.

I used to focus on collecting miles (mainly Aeroplan, BA and Flying Blue) but redemption fees (especially BA's obscenely high fees) and limited reward inventory have made miles in these reward programs less attractive to me. (Also, until recently, I had a very large stack of airline miles and therefore didn't feel the need to accumulate more through credit card purchases.) Instead, I've been focusing in the past year or so on hotel points because: a) there are practically no hidden fees attached to redemptions; b) several hotel programs have significantly reduced or eliminated blackout dates; and c) several credit cards offer status, or a leg up on status. In the first half of each calendar year, I focus on using my no-fee Hilton Amex (automatic Silver status, plus upgrade to Gold as soon as I achieve $20,000 spend in the year) and in the second half of the year (after I achieve Gold), I switch mainly to my Marriott Premier Visa (15 nights' credit toward Elite status, which is automatic Silver, plus a free night's stay certificate each year, for a $65 fee).

A lot of FTers prefer the Starwood Amex (in part because Starwood allows you to transfer points 1:1 to many airlines, with a 5,000 mile bonus on transfers of 20,000 points), but I find that Hilton and Marriott fit my travel patterns better than Starwood.

I also keep a Chase BA Visa (but hardly ever use it), because it provides for a companion ticket on premium BA fares (WT+, Club, First).