Incidently, the ICC calculates their peak Allrounder Rating by using the following formula. It's calculated at a single point in time;
(Batting Rating x Bowling Rating) / 1000
How do you rate all-rounders?
We have devised an all-rounder index that gives a good indication of who the best all-rounders in the world are in Test and One Day cricket. To obtain the index, simply take the player's batting and bowling points, multiply them together and divide by 1000. So a player with 800 batting and 0 bowling gets an index of zero (because he can't bowl and therefore isn't an all-rounder!), 600 batting/200 bowling gets 120, and 400 batting/400 bowling points gets an index of 160. An index of 300 plus is world class. There are far more all-rounders in ODIs than Tests, but the same names tend to appear high in both lists. Incidentally, this index does omit one important all-rounder skill, namely fielding. There is no satisfactory way of rating fielding skills statistically at present.
FAQ - Cricket Player Ranking | ICC Cricket
Any player with an Allrounder Rating of 300 or more is considered 'World Class'. Here are some examples that I looked-up. Unfortunately the ICC has not published an AT Highest list as it has for Batting and Bowling.
Sobers to Cairns round off the top 10 that I have found so far;
Garry Sobers (1967) = 669
Ian Botham (1980) = 646
Jacques Kallis (2002) = 616
Keith Miller (1952) = 573
Richie Benaud (1959) = 532
Imran Khan (1983) = 518
Tony Greig (1975) = 509
Aubrey Faulkner (1912) = 501
Andrew Flintoff (2005) = 501
Chris Cairns (2000) = 500
Shaun Pollock (2003) = 490
Richard Hadlee (1987) = 487
Alan Davidson (1961) = 474
Trevor Goddard (1967) = 469
Monty Noble (1904) = 451
Brian McMillan (1997) = 449
Kapil Dev (1981) = 433
Shakib al Hasan (2014) = 419
George Giffen (1896) = 418
Trevor Bailey (1957) = 417
Vinoo Mankad (1952) = 417
Daniel Vettori (2009) = 412
Wally Hammond (1937) = 386
Mushtaq Mohamad (1977) = 386
Stuart Broad (2012) = 382
Warwick Armstrong (1921) = 375
Ray Lindwall (1952) = 374
Sanath Jayasuriya (2005) = 356
Wasim Akram (2000) = 355
Frank Woolley (1913) = 349
Hedley Verity (1934) = 344
Malcolm Marshall (1988) = 341
Frank Worrell (1953) = 321
Ravi Shastri (1984) = 311
Wilfred Rhodes (1913) = 309
Chaminda Vaas (2008) = 300
Hugh Tayfield (1957) = 298
Maurice Tate (1929) = 286
Ben Stokes (2016) = 276
Shane Warne (2005) = 274
Peter Pollock (1970) = 259
Stan McCabe (1938) = 253
Bobby Peel (1895) = 249
Fazal Mahmood (1959) = 249
Anil Kumble (2008) = 242
Lance Cairns (1984) = 224
Dennis Lillee (1977) = 223
WG Grace (1890) = 207
Sachin Tendulkar (2001) = 205
Jim Laker (1956) = 202
Greg Chappell (1981) = 195
Viv Richards (1981) = 168
Derek Underwood (1974) = 160
Glenn McGrath (2005) = 110
Don Bradman (1933) = 56
Looking at that list - I reckon (as a guess) that anyone with an ICC Rating of less than or equal to 275 is a Specialist and anyone over 275 is an Allrounder.
Wilfred Rhodes is interesting. Yes he is an allrounder, but he is inferior to Lindwall, Jayasuriya, Wasim, Verity, Marshall, Worrell and Shastri.