Longest streaks of excellence (since 1969)

Most consecutive years while being ranked in the FIDE top 10 of all players:

28yrs, 6mo - Anatoly Karpov (Jan-72 to Jul-99)

25 yrs - Garry Kasparov (Jan-81 to Jan-06)

23yrs, 6 mo - Viswanathan Anand (Jul-91 to current)

21yrs, 6 mo - Vladimir Kramnik (Jan-93 to Sep-14)

This kind of consistency is apparently very rare since no one else is even remotely close to these four according to the ELO ratings adopted in 1970. The next highest I could find were :

13yrs+ - Lajos Portisch (1969 to Jan-83)

12yrs+ - Viktor Korchnoi (1969 to Jul-82)

10yrs, 6 mo - Veselin Topalov (Jul-01 to Jan-12)

Of the current players in the top 10 besides Anand, Carlsen has the current longest streak at 6+ years.