ROTF because it's an actual working sequel, not an exercise in ignoring the previous movie.

 

ROTF is lot better, at least got some cool action sequences , 7 per 10. TLK is completely unwatchable, story is a mess and no memorable action at all! 5 per 10.

 

RotF.
It had a somewhat comprehensible story, a few great scenes (FOREST BATTLE, FOREST BATTLE, and FOREST BATTLE), and some ok characters.
But it does have a problem nobody thought to point out above:
it made Optimus a monster.
In the '07 movie, it wasn't really that way. It just seemed like a slight amount of escalation. Oh, Megatron's gonna kill the s*** out of Jazz? I guess Optimus gotta kill the s*** out of Megatron. But in RotF...
"Give me your face!"
What's sad is that the Fallen was a somewhat ok villain. And Optimus, the most noble, right-hearted person in the multiverse (besides Shattered Glass), tears his fricking face off!
It made me want to scream!
But...
TLK is worse. By a long shot. I mean, they managed to screw up Unicron, who is such an easy villain that I bet a group of five-year-olds doing fan fictions could still make him awesome.
In Transformers: Prime, the twist that Unicron was inside of Earth worked. This was because they tried. But in TLK, it didn't. And mixed in with the s***ty plot, it made no sense.
TLK isn't just mediocre. TLK isn't just horrifying. TLK is a blasphemy to the franchise.
I mean, they had to make a G1 film to correct that stupidness!