Hard to go past Beefy, be interesting to look at Richards career split into halves
With Richards it's more a case of 2 short periods of superlativeness (1977 and 1979/80-1981 - he averaged 90-odd and ~80, from memory, respectively) amongst what was mostly (1974/75-1988/89) a career where he failed to reach the heights his many admirers expected.
And the truth is that Botham's case was no rapid decline, not in all-round terms and not any more than the usual stages of a player's career. Botham was still easily a Test-class cricketer until 1987 - he only played a few games in 1989 and another few (very ill-advisedly) in 1991, 1991/92 and 1992.
He was superlative beyond superlative 1977/78-1982, especially when not captain; he was still good (much more so as batsman than bowler) when not playing West Indies 1982/83-1987 (was diabolical against WI) and as I say, way below Test-class from 1989 onwards. It wasn't a case of an overnight decline from sensational to awful.
For an example of something like that, look at Allan Donald. Though he was at the stage of his career when that sort of thing can happen, especially to cricketers like him - seam-bowlers who rely heavily on physical power. But it was amazing to see a bowler who'd been sensational for his entire career 1992-2001 turn-up in 2001/02 so badly a shadow of his former self. I've rarely wished that a cricketer hadn't attempted to play more than I did there.
Those who suffered a comparable decline mid-career are rare, but one example is the relatively little-known Trinidadian Tony Gray. In his first ~4 years he took about 600 First-Class wickets at 20, including 5 Tests where he averaged 17. Then he suffered a serious injury and for the rest of his career averaged 29.
And almost all such declines can be put down to a serious injury, really. To see one with no apparent obvious cause is exceedingly rare.
