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Of these there are many.
Working out who to list was tough, because there are so many drivers out there with impressive records or who look very talented.
And it's worth mentioning that, despite not making my five, most of the guys listed below could probably do a better job than one or more drivers currently in F1.
Davide Valsecchi is the reigning GP2 champion and current third driver for Lotus.
He's undoubtedly a very good driver, but isn't in my Top 5 because of his lower formulae record and the sheer amount of time it took him to claim the GP2 crown—he started 96 races.
Luiz Razia came second to Valsecchi, but before that he was rarely eye-catching. He started 101 GP2 races.
Fabio Leimer currently leads the GP2 championship and was close to being included. He has a decent lower formulae record and usually beats his teammates—but like Valsecchi, his success took an age. This is his fourth full year.
Sticking with GP2, early 2013 season leader Stefano Coletti appears to be talented despite his mediocre record. He is a fine example of a driver whose history was affected by never driving for the best lower-formulae teams.
But for me at least, there remain more question marks over him than the five I picked.
Mark Webber's protege Mitch Evans looks very promising, but at only 19, he could do with at least one more year in GP2—hopefully with a front-running team.
The same can be said for Red Bull Junior Daniil Kyvat (but he'll probably be in FR3.5).
Ferrari's Raffaele Marciello is another up-and-coming name, but he's got at least two more years in the small leagues before he'll be ready.
Robert Wickens is currently plying his trade in DTM. He beat teammate Jean-Eric Vergne to the FR3.5 title in 2011, but he doesn't stand out as strongly as some others. Maybe if he'd been able to stay in single-seaters...
Sam Bird is another prominent name missing. He's looked very impressive in GP2 this year, after a good showing in FR3.5 in 2012.
But other guys have had more success in the junior ranks. The old faithful gut feeling put him below the rest.
Sergey Sirotkin looks set to step up in 2014 if Sauber can get him a Super License, but he isn't ready yet. It remains to be seen if he ever will be.
And finally, Antonio Felix da Costa. I didn't even consider him as he's nearly guaranteed a seat at Toro Rosso next year, but my gut feeling says he wouldn't have made the Top 5.