I just did this in response to a question at another site, and I thought I'd post it here.
First, getting into the Top 3 is pretty important. Eric Hosmer (the #3 overall pick) has a good chance to not sign. If he doesn't, the Royals will get #4 overall in compensation, so if the Giants have the fourth worst record, they'd still get the #5 overall pick.
As it stands, the Giants have the 4th-worst record in baseball. San Diego has the third worst, four games ahead of the Giants. Seattle is 4.5 games ahead of the Giants, while Washington is 5.5 games ahead of the Giants. On the other side, Cleveland is currently 5th-worst, two games behind the Giants. It should be noted the Giants have two games in hand on San Diego, and one game in hand on both Seattle and Washington.
So, here are some players who some think will be in the Top 5 picks.
Stephen Strasburg, RHP, San Diego State
I don't think this guy needs any introduction.
Mychal Givens, SS/RHP, Plant HS (FL)
Givens has been anointed the 'next' version of Tim Beckham by BA, which may be too early. Givens throws mid-90's, and has a 6.79 time for the 60. Heck, he even has time at catcher (look out Posey). However, as a high schooler, there's a lot of projection and risk.
Kyle Gibson, RHP, Missouri
Fastball in the 1990's with a good slider as a sophomore. He has very good control and picture perfect mechanics, and is very projectable. Probably is one of the safest picks in the draft.
Matt Graham, RHP, Oak Ridge HS (TX)
Graham is one of the hardest throwers in the draft already, and this comes from a guy who is scouted only as a junior in high school. Has a lot of work on his offspeed pitches to do, however.
Alex White, RHP, North Carolina
Mid-90's fastball, but he works better with a splitfinger in the low-90's. Big righthander who works well with changing speeds. Probably not a big strikeout pitcher in the future, but has the stuff to be the next Brandon Webb.
Dustin Ackley, 1B/OF, North Carolina
He's been one of the top hitters in the ACC since he was a freshman, and will compete with Givens to be the top offensive player in the draft. However, his power is suspect, and he won't be a 40-50 HR guy...he'll have to develop to get to 25. But he has the approach to work that. He's a good defensive 1B, and may be a good corner OF as well.
Matt Purke, LHP, Klein HS (TX)
Purke will compete with Graham to be the top high school pitcher. He doesn't have the velocity, but he has very good control and a Low-90's fastball, so he has plenty to work with.
Donovan Tate, OF, Cartersville HS (GA)
Tate is a top football player, but is expected to choose baseball. He has the range for center field, and a good arm. Scouts like his power, but like Fairley, there's questions about his contact hitting.
Jacob Turner, RHP, Westminster Christian Academy, MO
He has a low-90's fastball, but his 12-6 curve is his calling card. At 6'4" and 205 pounds, his body should be as prototypical as you can get for a pitcher. He has the intangibles to continue to improve.
Austin Maddox, C, Eagle's View Academy (FL)
Maddox is the best catcher in the draft, and throws low-90's as well. He has great bat speed, but still has to improve his production to a degree to be a Top 5 pick.
Grant Green, SS, USC
Green is the longest shot of this group to make it into the Top 5. He's got the range and arm to stick at short, he has quick hands, and the speed to work with it. However, he does not have the power or overwhelming line drive bat to be a premium prospect, even at short.