Picking a young rider to follow is always a bit tricky. Like most sports, there are more riders hyped as the next Armstrong/Indurain/Mercx/etc... But living up to such expectations is, of course, rare. And, again, these days, you risk being disappointed by a doping scandal. But, it's still great to follow the sport and watch new talents emerge.
Two of the three Grand Tours have a classification for honoring the best young rider (under age 25), so that's a good place to start.
For the 2007 Tour de France, the best young rider (white jersey) was also the overall winner of the race (yellow jersey) -- Alberto Contador of Spain, who raced for Discovery Channel Team. His was an impressive performance, marred somewhat by the doping scandals that eliminated a number of other contenders during the race. But, he rode courageously, aggressively, and tenaciously. So, it was fun to watch him.
In the Giro d'Italia, the best young rider was Andy Schleck, who also finished second overall in the race. So, again, another grand tour where young riders placed high overall.
To my knowledge, the Vuelta a Espana does not have a best young rider classification.
Here's a link to the final standings in 2007 for the Best Younger Rider in each Grand Tour:
Tour de France:
http://www.letour.com/2007/TDF/LIVE/us/2000/classement/index.html
Giro d'Italia:
http://www.cyclingnews.com/road/2007/giro07/?id=results/giro0721#gc
Contador and Schleck are hot names to follow these days.
For many years, Tom Danielson was hyped as the "next Lance Armstrong" by some. Now 29, Danielson has won some races for Team Discovery, but has never really placed well in a grand tour. He signed this year to ride next year with Team Slipstream, so we'll see how that works out. American fan favorites Dave Zabrieski and Christian Van de Velde are also racing next year for Slipstream. Zabrieski is also 29, is the reigning US Time Trial Champion, and wore the yellow jersey for eight stages of the 2005 Tour de France, before crashing out.
Plenty of choices, as you can see! Just start watching and so who wins your heart...