Beast Wars is the series to watch.
Beast Machines is a possible watch depending on how you feel after BEast Wars.

Animated is the best series after Beast Wars, but it has a stylised aesthetic that is not everyone's cup of tea.

I also recommend the last 15-20 episodes of Armada (don't need to see the preceding 30 episodes) and Cybertron and RID/Car Robots if you can get them, but neither series is a must watch.