Interesting topic. From the top of my head most of the somewhat number twos of the past few decades:
– Gerhard Berger. Had a very successful career, but was never in a proper title fight. Had no chance to beat teammate Senna in 1990 and 1991, and joined Benetton with title hopes in 1996 just when their form took a dive, the team going from WDC and WCC in 1995 to a winless campaign in 1996.
– David Coulthard had a chance with both Williams and McLaren. Closest chances were probably 1999 (when he usually did everything to sabotage Häkkinen’s races while losing to Irvine) and 2001 (but could not stop Schumacher).
– Rubens Barrichello. Of course, Rubens wasn’t allowed to win while Michael was the number one, but had a chance with Brawn as well.
– Mark Webber. Again, without Vettel, might be a champ.