WHICH IS BEST SUITABLE SPEED CONTROLLER FOR A CONTAINER WHICH HAS TO BE ROTATED AT THREE DIFFERENT SPEEDS OF 30 40 AND 50 RPM.

A lot depends on a number of factors here. 1) Is there a gearbox? 2) What is the required torque to rotate this container? Use worst case scenario for calculations. 3) Acceleration and Deceleration times?
Once these three questions can be answered selection of the speed controller and motor can begin.
If there is a gearbox between the motor and the container I suspect a VFD in closed loop mode and an AC motor might be best or least expensive.
If there is no gearbox and the required torque is not tremendous one might be able to use a Servo motor and controller. This option will require more precise calculations for load and speed.
I would not use a DC motor and controller here. First fairly old technology, second the power factor for DC controller is not great. Third the DC motor requires more maintenance than the AC or servo motor.
I think your best option here is to use a VFD, AC motor with encoder feedback and 40:1 or 50:1 gearbox.

Good Luck
Jack

thank you

I observe that you are from College of Dairy Technology, I suggest a cautious approach. If the load is varying due to sloshing of liquid contained in the vessel and the liquid solidifies gradually with time of process, then you can expect a varying unbalance in load with respect to speed, time, solidification process etc. and tuning of a drive is quite an uphill task especially with the above brief varying load conditions. Age old drives with Fly wheel in the transmission used to answer quite effectively. DC drives also use to perform ok due to armature reaction etc. characteristics of the DC motor. Eddy current was a failure in such applications. I am of view AC drives with good PID tuning with proper drive rating selection and MOI focus should give you an good results. Of course gear box deployment is mandatory for good engineering practice and optimization of drive rating.