It depends on what you expect x to be
If x is a boxed int then (int)x is quickest.
If x is a string but is definitely a valid number then int.Parse(x) is best
If x is a string but it might not be valid then int.TryParse(x) is far quicker than a try-catch block.
The difference between Parse and TryParse is negligible in all but the very largest loops.
If you don’t know what x is (maybe a string or a boxed int) then Convert.ToInt32(x) is best.
These generalised rules are also true for all value types with static Parse and TryParse methods.