Hmmm.... nice question. And to answer honestly, I'm not quite sure if the electric organ is really the BEST of all time really. But, it's definitely up there.
And for the sake of answering this question I'm going to assume that the Leslie is implied everytime you mention the word "Hammond" as no one really wants to hear a Hammond Organ without a Leslie speaker...despite Laurens Hammond's ironic attempts to make his organ "Leslieproof" and thwart Don Leslie's attempt to enhance the Hammond Organ.
Let's just say for the sake of argument that we only consider the best instruments of the 20th century. Then the best the electric organ will do is tie with the electric guitar for first. And if we consider instruments of all time, the electric organ would either be 2nd or 3rd to strings (i.e., violin, viola, cello, etc.) and to the actual Pipe Organ.
However, if we start dealing with the best BRAND/MODEL of instruments then I think Hammond and the B-3/C-3 models have it covered. Out of all of the various brands and models of various instruments throughout history [i.e., Stradivarius (violin), Gibson Les Pauls & Fender Stratocasters (electric guitar), Cavaille Coll (pipe organ), etc.] the Hammond is the only one that has spawned a whole genre of music, jazz organ. I say that with confidence as no one has heard jazz organ on anything other than a Hammond or some electric instrument impersonating one (except maybe some early experimental Fats Waller on a pipe organ, which was reeeally bad). And no other brand/model of instrument has spawned so many immitations, which is ironic considering the Hammond Organ was originally intended to be an affordable & realistic immitation of a pipe organ.
So, if that's what you're saying then yes, I agree. The Hammond Organ is the best brand/model of any type of instrument in the 20th century; though, it may be tied for best with the Stradivarius violin in the contest for best of all time.
As for best instrument I guess you'd really have to say the piano since the arrangement of keys on it's keyboard was the one of (if not) the most important achievement in musical engineering of all time and the basis for every keyboard instrument thereafter...including the organ.
Where do you play? I play in the southeast area of Michigan. And do you play gospel or jazz? If your name is short for Jimmy Smith (R.I.P.), I'll assume it's jazz.