History of Slow Left-arm Orthodox (SLAO) spin is probably the oldest in cricket along with batting & fast bowling. In fast 50 years of Test cricket, some of the greatest olden days' SLAO spinner played for ENG (Peate, Briggs, Peel, Blythe, Rhodes), of whom I have no idea or haven't ever seen any recording. Some of them must be outstanding with their control at least, because despite low scoring matches those days, an average under 20 & economy of 2 is outstanding.
In last 100 years (after WW 1), I think the best 3 had been Verity, Underwood & Bedi. Valentine, Qasim, Herath, Wardle (he could bowl both variants of orthodox & chinaman) & Lock (alleged with a bit dodgy action, haven't seen though) must have been very good. For a good few years, Manainder & Dulip Doshi were very good, but I guess due to lack of variation, they din't survive longer, just like A Rehman.
Underwood has one of the best averages among modern spinners (of all variants), because of his outstanding economy BUT, he was extremely condition dependent bowler, on wet or drying wickets, it was near impossible to score offr Derek, but I don't hink he was that effective on good, firm wickets or in 1st 3 days of a Test. BS Bedi was probably the most classical of the SLAO spinners - with his flight, loop & armer, even on best of wickets Bedi was outstanding. Zaheer & Javed dented Bedi's career big time in his last few years, otherwise he would have finished with a career average close to Underwood. I think, among all spinners, Bedi & Qadir troubled Viv the most. Verity was probably a mixture of BS Bedi & D Underwood - an almost unplayable bowler on "sticky-dog", but equally effective on good wickets. He passed away, probably at 38 & played his last Test 3 years back - those were days when spinners easily played FC till their late 40s & Test till early 40s. Had WW II not robbed his best 4/5 years; I think Verity would have finished as the then highest wicket-taker in Test (Before Trueman, Grimmet with 236 was probably the record holder).
In modern days, Herath actually showed what he could have done had he not been sub-pressed by the presence of Murali; however, I am a bit reserved on Hereth because of his superman efforts against PAK, who oblate had been a real bunny of SLAO spin & their dumb, incompetent, clueless & retard selectors fed Herath with 11 right-handers to target a spot whole day long, years after years (I don't think they noticed that 2 of the top scores 236 & 168 against Herath were by Taufique Umar & Fawad Alam). Panesar had a great start, but he is probably the bowler version of KP - self destructive & a management headache, otherwise could have broken Underwood's English record.
There was one more guy - probably one of the best ever cricketers never to play Test Cricket. For almost 3 decades, Rajinder Goel was the top spinner in Indian domestic circuits, but he was never picked over Bedi. Unfortunately, both born on same time (probably same year) & Bedi made the early reputation on his debut, never allowed Goel to dislodge him. Indian selectors often played 2 off-spinners together, but never tried to SLAO together in 20 years & Bedi had his own strong lobby (once he was dropped on disciplinary grounds, but selectors deliberately made Goel 12th man, so that his performance doesn't make it difficult for Bedi's return). I think, Goel had better FC stats for most seasons over Bedi, but Bedi was preferred for his variations - on under-prepared track, I don't think had ever been a better spinner than Goel - he could hit a coin on good-lenth spot whole day with his accuracy.
If I am to rank, I probably 'll go with Bedi, Verity, Underwood & Herath in that rank.
