After a certain age people stop asking you what your favorite dinosaur is, and I think that's sad.
My current one's the Anchiornis, because it's in the same clade as birds so it's in their family tree, and it really looks like a prototype of a bird. It had 4 wings for example, but it already looked very birb-y:
(Figured I'd just turn an earlier comment into an actual post because why not)
Patagotitan mayorum. I think that it's the largest that we know about - around 40m large. Herbivore, I can't help but think on it as a dino-giraffe: eating leaves, nesting its eggs, not giving too much of a fuck about the small critters nearby.
I love this question! This made me really happy. My favorite has always been (and remains) the brontosaurus. (Probably can blame "The Land Before Time" for its influence.)
I remember in the early/mid-90s being corrected that brontosaurus wasn't a dinosaur species, but was actually the same as apatosaurus. Being a stubborn child, I refused to accept this and always considered the brontosaurus, not the apatosaurus, my favorite. I felt so vindicated when the study came out in 2015 that brontosaurus and apatosaurus weren't the same.
Perhaps that's why I still stubbornly refuse to let go of Pluto: the minute hope that my favorite planet will be recognized again someday. Alas.
If we are accepting non-dinosaur but ancient creatures, my favorite is the anomalocaris :) it's just a goofy lil guy with a mouth snoot. Plus, they never say no to a snack. (They're at the end.)
Stenonychosaurus (née Troodon) because as unlikely and silly an idea as it was I always found the whole "how intelligent could they have become if they hadn't gone extinct" thought experiment fascinating.
I also have a soft spot for Struthiomimus just because it's such a fun word to say.
Argentinosaurus. Biggest Tree Star eater ever. I think it would be the coolest thing ever to like, mount a dwelling on its back, live on and take care of it.