Ive played ALL Assassins Creed game, I stoped after revelation, and some years after started again from 3 up to when odyssey released
1 is bad but it was the first so it has a slight pass, Revalations is the worse of the Ezio series, but it isnt as bad of some of this list, AC Unity is lovely to look at, and the french revolution is a super interesting period of history, with amazing architecture, but the mission tructure and order (fucked up by multiplayer) and singleplayer are a fucking mess (multiplayer killed that game). Origins is an actually loved game so i dont know why its in the list (and its great), it should have been AC Rougue which actually has some redeeming qualities...
And Syndicate is actually fun and sadly forgotten, with a great city to visit and some great WW setpieces just to use famous world building that were not there in Victorian times (the only thing it fails, is cutting the map just before you can visit the Crystal Palace), and the two characters are really fun so that gets my vote.
But as the OP wants to defend so behemently AC3, I will say it the absolute fucking worse AC ever. Its absolutely terrible, the only saving grace is the ship battles, and those are better in Black Flag. It has zero personality, american "architecture" and cities are fucking bland in 1770 (if you really want to do an AC game set in USA, use new york during the 1890 to 1930 period), to the point the game tries to make you travel half of the game through the fucking sewers or going above cloned trees. The War of Independence is fucking boring (the other historic periods in the series are more interesting). The main character is nearly as bad as Altair (and i hate one of our best voice actors, Ivan Muelas, was wasted on him). The modern part is... absolutely atrocious. Oh you need to go to a football stadium in Brazil. Will that be the Maracaná? which would be super fun to climb? NOPE, undescriptive stadium that you dont even really climb to see well. Oh you are on new york, are we gonna climb the empire state, maybe the chrysler? NOPE, a fucking undescriptive building under construction.
And the list of bad boring things on that game goes on and on and on...
Fucking 3 made me stop playing the series I love (though revelation didnt help), and when i got years after and tried again becuase i really wanted to play oddysey, and forced myself to beat that fucking thing, I was incredibly scared of jumping to Black Flag after (was marathoning the series) becuase i thought it was gonna be equally crap, thankfully it was great fun, even when i thought the time period wasnt gonna interest me that much becuase of the architecture, and i was pleasently surprised i did.
Thats how bad AC3 is.
Valhalla is as boring, becuase im fucking bored of Vikings and should have been Rome, so im not a fan of the last game either. Though i wouldnt call it atrocious or bad, like AC3 is.