Medication is a last resort. Like, way far away last resort. I know all about tantrums - my almost-5-year-old has autism, which one of the hallmarks is uncontrollable behavior and extreme tantrums.
You say you've tried consistency, but then you say, you've tried time outs, spanking, and other consequences. How long did you try consistency? A week? A month? Consistency works, but it doesn't mean trying the same thing short-term. It means the same result for the same action. Every time. Forever.
Having dealt with terrible behavior for years, we're through the worst of it now. There were days when my son was in time out over and over again, for hours. It wasn't fun. There were days I felt like I wanted to run away, or give him up, or drink myself into a coma, but I stayed, stuck with it, and made it through.
I know it's hard, but if you get him to a place where he knows with 100% certainty what will happen if he acts this way, he'll learn to self-regulate.
I have gone through and through w/ this type of stuff with my daughter. It's tough, and it might even depend on how your family works for what might be causing this behavior. Does he have any other siblings? Does he get enough sleep? Are you 100% consistent in your warnings and rules (the slightest slip up, he realizes he may be able to get away w/ more)? Are you in sync w/ the babysitter for disclipine measures? Do you give him simple chores so he can learn responsiblity? Do you practice self-control w/ him and reward good behavior?
There really are numerous things that can result in bad behavior. I would suggest talking to your pediatrician, or even friends/family that see you and your family daily that may have a better suggestion... I know when I have issues w/ my kids I go to my friends/family first b/c they know my situation best and can weigh in better.
You may want to consider taking away toys and such that are very important to him. At this age, play time, toys, tv, etc seem to be important to kids in this day and age. When my daughter gets stubborn, we take away the things that are most important to her (like playing outside) and she is a whole new child the next day :) Of course, my daughter is now 7 years old, but this has worked best even since she was a toddler. When she acted out in public as a toddler, she was no longer allowed out w/ Mommy to run errands or go to fun places. Try to think of what makes him happy- getting out and about, playing w/ a certain special toy, a special thing you do for him as a treat... and use that as a means for punishment. My daughter did not do very well with spankings either... timeouts didn't always work, but taking away things she loved was the best discipline for her.