A Spinner is someone who is capable of turning the ball by effectively rolling his fingers across the seam.
A Spinner can turn the ball mostly over the dusty and dead pitch where the CRACKS on the pitch could exaggerate the normal turning ball to turn and bounce like a Spitting Cobra to create discomfort to the baBestsman.
A Spinner can be useful to a captain when the pitch has a little bit of spice in it especially during test matches on 4th or 5th days based up on the conditions.
A Spinner can occasionally get the arm ball in place/the straight one which might confuse the batsman
If the conditions does not favor, a Spinner could turn out to be a victim at the hands of opposition just because they have enough timing to hit the ball which has no speed and spin

Best Spinner :

A Best Spinner is always considered as an asset to a captain and his team
A Best Spinner can always turn the ball for you in all the conditions right from dusty pitches to the green top wickets
If the condition does not favors it, a Best Spinner could soon turn out to be a line and length bowler who could bowl in good line at consistent basis making sure that he gets a wicket or two or at lease not to leak runs
A Best Spinner never always reliant on a single technique, and always keeps a lot of variations in his hands which could be utilized on various situations
At last A Best Spinner tries to read from Batsman’s point of view and try to restrict the scoring options

Upon considering the above criteria I could think of only one Spinner who could possibly check all the boxes. Keeping aside the stats, the best Spinner till date is

Shane Keith Warne
Reason :

Shane Warne is someone who came from Australia where the wickets does not assist much of spin and yet he proved himself turning the ball at various conditions across the country

Shane Warne is capable of drifting the ball through the air. He will bowl over the wicket to a right handed batsman, the ball will drift towards the leg side, the batsman will expect the ball to go Wide on the legs and then the ball pitches and turns steeply towards the off stump making the batsman clueless. Who can forge the Ball of Century bowled by Shane Warne to Mike Gatting ??

Shane Warne never just relied on his leg break technique. He got a drift ball, arm ball, quick pitching straight ball, which he managed to bowl on both over and around the wicket
Shane Warne is known for his accuracy. World has produced more talented spinners who lost their way just because they are too greedy about turning the ball and not bowling the right line and length. But Warne was a gifted spinner who has a good accuracy that he could bowl anywhere based up on the captain’s request
Sensible field setting is one of the greatest assets of Shane Warne. When he comes to bowl he always not relied on captain to set the field. He comes forward to set his own field and bowl to that field. It might seem easy to talk but trust me it needs a great vision and tactical skill to set a particular field
Finally, Shane Warne is someone who is known to be aggressive on the field. When the batsmen is going after him, he would just pretend to set aggressive field setting by bringing the fine leg/deep mid wicket to come closer and move backward just to create an element of doubt on the batsman’s mind.
I hope these are the qualities which are like an add-on for a spinner and think about how relaxed a captain would be if you have a spinner like Shane Warne on your side