As I see this, "history" refers to all gens from 1 to 5 and ttar didn't exist in gen 1. So depending on how you see it, you are not counting ttar's performance in gen 1 and only in 2-5. This is like if I only count Lax's performance in 1-4 gens which is better than its 1-5 performance and at least on par with ttar's in 2-5 gens.
And then, if I go further, the most dominant pokemon, behind this criteria, is simply the best BW pokemon that appeared in gen 5. Lets say this pokemon is top 5 in BW; a #5 is better than Snorlax's average of #2, #1, #10, #40 and #60 (or whatever it is) in gens 1 to 5 respectively, and probably better than ttar's (gens 2 to 5) average too.
Honestly, I think the "most dominant pokemon ever" should be picked from the first 151 mons. I'd just add up the expected usage percentage of each pokemon in each gen, and the mon with the highest result gets the title.
(for snorlax for example 95%+100%+30%+6%+4%=235/500=47%)