For me, I think it is this, bar none. When it came out around a decade+ ago, there were even arguments that is was official. (Of course, I think it surpasses official art.)
(http://s14.postimage.org/duf3e5tzl/aeka_in_juraian_stasis_gear_2.jpg) (http://postimage.org/)
upload pictures (http://postimage.org/)
Among the images that I have available for linking:
This one for Ayeka: http://img.photobucket.com/albums/v430/admiraltigerclaw/aeka090ce4.jpg
This one for Mihoshi: http://achibner.deviantart.com/art/Commission-Mihoshi-82912260
This one for Washuu: http://pewpie.deviantart.com/art/Tenchi-Muyo-Washu-289716934
This one for Ryoko: http://spiritamael.deviantart.com/art/Portret-of-Ryoko-16743025
This one for Tsunami: http://laindragon.deviantart.com/art/Tsunami-65800846
Of the ones I've seen, this one seems to be the best. ;D
http://www.advancedanime.com/pictures/1772923.png (http://www.advancedanime.com/pictures/1772923.png)
Simply awesome!
http://tr3s-sss.deviantart.com/#/d5qmkko
(http://s14.postimage.org/7ikkuk1u9/aeka_stand_alone_by_tr3s_sss_d5qmkko.jpg) (http://postimage.org/image/518tnahxp/full/)
King Tenchi.....no contest.
Even using it for myanimelist background.
http://myanimelist.net/animelist/Tenchi_Ryu&status=1&order=0