I haven't bothered to see it yet, but I hear Transformers 3 is pretty painful to watch.
It's a considerable improvement on 'Revenge of the Fallen', so definitely not the worst of the live action trilogy.
I would like to vote for 'Blade: Trinity'. 'Blade' was good, 'Blade 2' was awesome. 'Blade: Trinity'... bleh, it's painful to even think about it.