9. Finis Valorum
While Valorum does briefly appear in The Phantom Menace (played by Terence Stamp, no less), he's such a minor character in the saga that it's easy to forget his significance.
While his importance to the narrative undoubtedly comes from the lack of faith the Senate had in Valorum, the roots of the problem are actually much further reaching than all that.
Valorum was always a reasonably fair politician, although despite his well-meaning nature, he appeared to have a streak of disastrously bad luck. This apparent streak of misfortune was rooted in the Chancellor's overly trusting nature and his poor head for politics, as one of Valorum's trusted friends (and occasional advisor) was none other than Sheev Palpatine, the secret Sith lord who would later become the Emperor.
Valorum's soft nature and slower approach to things proved to be his downfall, paving the way for Palpatine to overthrow him, seize his power, and go on to dissolve the Republic entirely.
The saddest part is, if Valorum had been able to wise up to Palpatine's schemes, he could easily have exposed the would-be tyrant long before he ever had a chance to wreak galactic havoc.