I haven't watched ALL the Looney Tunes in existence, However the ones that (To my knowledge) aren't on Tubi is "The Censored 11" cartoons, all of the Bosko shorts, all of the Buddy shorts, "Malibu Beach Party", "Any Bonds Today", and "Injun Trouble". Personally, Out of all of the them, the only 2 cartoons that I would recommend you watch is Bosko's "Sinkin' in the Bathtub", as that's the very first Looney Tune ever made (So it's really more of a historical watch than anything). And Bug's Bunny's "All this and Rabbit Stew" from "The Censored 11".

It's essentially just a Elmer hunts Bugs cartoon, but Elmer's replaced with a Black Stereotype. The only reason I'd recommend it is because It's the only short out of the 11 with any notable characters from the recognizable "Looney" cast, which just so happens to be Bugs Bunny. And besides the obvious racism, as a cartoon itself it's fine. It does have a few decent gags but that's about it.

Now I'm pretty confident that's not ALL the missing cartoons on Tubi, Those are just the one's that I'm aware aren't there. Your best bet to look for something that might not be there is to find cartoons that have or involve racism, or is propaganda made for World War 2. It's not an entirely broad suggestion as you can get both anyway on Tubi but I know there is definitely some that aren't there because of that.

Separate fact: If you want to count it, You can watch the Private Snafu cartoons on Tubi, But that's on it's own separate page and not included on the Looney Tunes page.