I would go with Lloyd as the worst case of acting in the prequels. I've found that there are generally two types of child acting. One is comatose. The kid speaks in quiet tones and shows maybe one look throughout each scene (happy, sad, excited, etc). Examples would be the kid from Superman Returns and Daniel Radcliffe from the first Harry Potter. The second, which Lloyd belongs/belonged to suffers from something I've dubbed "school play" syndrome, where the kid has gotten so used to shouting his/her lines that they carry it over to film roles. And that's what Lloyd's performance consists of. He over enunciates words and his only motivation is to make sure everyone hears what he's saying.

The best actors from the Prequel Trilogy were undoubtedly Liam Neeson, Ewan McGregor, Ian McDiarmid and Christopher Lee. You may have noticed they're all British. Liam brought much of the heart and warmth of The Phantom Menace and he was missed after his departure. Ewan has done a great job at both working Alec Guinness into his performance and also making the role his own. However, I think it shows in Attack of the Clones that he was a tad uncomfortable acting with characters who weren't there. Ian McDiarmid was great as Palpatine. Of course, when he turned Emperor, things got wacky, but I always felt his performance in Return of the Jedi was ridiculously over the top. Christopher Lee was great, but I think his performance had more to do with his commanding presence and voice than anything else.

I guess that leaves Hayden, Natalie, and Samuel L. I bear no hatred for Hayden's performance so I won't say he was awful in the role. I think it has more to do with the lack of direction or perhaps what Lucas wanted for the role. In rewatching Titanic, I noticed that Leonardo DiCaprio's performance was similar in a way. Neither he nor Hayden are particularly great in the respective films. They're both given horribly clunky and stilted dialogue and neither really make them sound convincing. Still, they seem to service the characters and I think they work for the film. I thought Natalie was great in Episode I, but she seemed really stiff in Episode II. Not just the readings of her terrible dialogue, but also in her body language. I felt that she improved in Episode III, but her lines were definitely more of the corny variety. Nevertheless, I think she managed to deliver a heartbreaking performance in the films. Samuel L. Jackson was just Samuel L. Jackson. In Revenge of the Sith, the part just seemed to call for Windu to act bitter and tired most of the time. I guess that's the Clone War for ya. The character never quite worked and Jackson just seemed stiff in the part. Is it Lucas' fault? Perhaps, but Jackson did campaign for the part, so I hoped he would've done more with it.