For me TOS will always have a special place in my heart, due to when I watched it, but I wouldn't say it is the best Trek (as the poll question assumes).
For me the best trek of all is DS9. TNG is second, though it is light-years behind. VOY takes third place (mainly due to the first three seasons, after which it went downhill). ENT is last, by the time they managed to do something decent with the show it was already over.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)