You might enjoy both books of Bach's Well-Tempered Clavier on harpsichord in Bach/Lehman 1722 temperament (from Bradley Lehman, A.Mus.D.), which feels to me like what Bach really used. You might like this temperament, too!
If there is an immediate emotional difference between the following two versions of Prelude in C major from Book I of The Well-Tempered Clavier by Bach then, good!
Bach in equal temperament
Bach in Bach-Lehman temperament
Otherwise, your MIDI setup is not right somehow. You shouldn't bother to download books I and II in their entirety.
Available zip formats:
the major keys (the order is F C G Bb D Eb Ab F# A C# B E)
the minor keys (the order is D A E G C B F Eb G# F# Bb C#)
To start playing after downloading, just open the folders, select all the MIDI files, and drag them en masse to a RealPlayer window.
Toward answering the question, 'Which keys sound brightest?' here the pieces in the Well-Tempered Clavier are in order of increasing 'brightness': the size of the tonic chord's major third, essentially, after dividing major keys from minor.
For sizes of major thirds and fifths in Bach-Lehman temperament, see this chart.
For the detailed sorting decisions, see 'readme.txt' inside the major and minor key zip files.
I tuned the MIDI files to Bach-Lehman temperament using the program, Scala.
(For this, it is essential to use good wave-table MIDI synthesizer software like the Microsoft GS Wavetable SW Synth (which is based on Roland wavetables). The Creative (SoundBlaster) SoundFont Synth and the QuickTime Music Synthesizer (at least on PC's) somehow blot out temperaments' emotional content, and thus they make meaningless this listening comparison.)
Alan Kennington and Yo Tomita have given me permission to reuse their MIDI realizations.
Copyright (c) 2010 Mark D. Blackwell.