They couldn't touch the Flash character because of the legal wrangling about a potential movie, but they really handled it well by giving us Bart Allen instead. The two episodes that have seen him in real action have been good ones.
The thing that sold the role to me was the fact that they made him much faster than Clark. That was a touch I enjoyed seeing. I was afraid they'd make it some close race so their main hero would still seem unbeatable. Instead, they gave us a guy who was better at running than anyone else...just as it should be.
Zatanna is one of the characters that was closest to their actual comic counterpart than any other. The costume was absolutely dead-on. Her powers were just as you imagined they would be. And the actress they chose for the role couldn't have been improved upon. Serinda Swan looked like the model they based the comic character on.
I've never been a big fan of the character in the comics (and that stupid elf costume she had in the 70's was pathetic), but after the events of Identity Crisis and her role in mind-wiping Batman I had to give her some credit. Smallville handled her with a healthy respect that kept her from becoming as cardboard a role as the Wonder Twins in a later episode.
All right, so he wasn't bare-chested and beefed-up like the comic book version, but you have to give Smallville credit for what they gave us. He had wings, and the helmet was dead-on. He had that brooding quality we were expecting, and those conflicted moments with Green Arrow were a nice touch.
Yes, he's dead now in the series (as of this writing), but he always comes back in the comics.
We only got him for that one double-length episode, but give the writers credit where it's due: they nailed the character. Ok, the crazy secret identity was dumb. Still, when he was a hero, he was absolutely dead-on.
I still have no idea how the actor saw out of that helmet with the lights in his eyes. My biggest complaint is the fact that they only kept him around for a single adventure. It would have been nice to have seen a bigger JSA reunion later on. I know they couldn't touch Green Lantern or the Flash because of movie possibilities, but Wildcat, the Spectre, and even Doctor Midnite or Hourman could have filled the void to complete the team roster.
Another character from the comics that I wasn't a big fan of, Aquaman turned out pretty cool in his guest shots.
This was another character that the writers were willing to concede was faster than Clark at something. He could outswim the guy with no effort. Again, this made sense and it was good to see the writers give him this edge.
His first episode was so great, but his guest shot in the Justice League episode the following season let him stand out. He's appeared since then a few times. Unfortunately, the Aquaman character himself is pretty useless without water around, so he's been relegated to brooding shots in the background.
Love him or hate him, you have to admit it's pretty nice to see the Emerald Archer in action. Letting him use a compound bow and even Kryptonite arrows (in the ninth season) showed a lot of detail to attention.
While I feel the yellow goatee is essential to the character in the comics, I don't think Justin Hartley would look right with it.
I don't know if you've seen Hartley's portrayal of Aquaman for the failed series "Mercy Reef", but I thought he did a great job with it and I would have enjoyed seeing the series take off...especially in Smallville's upcoming absence.
The Legion of Superheroes
We couldn't get the whole roster, so they gave us the three top stars in the team. Cosmic Boy was played out nicely, as well as Saturn Girl. I wasn't as impressed with the Lightning Lad character. I guess it was hard to visualize him married to Saturn Girl as they later became in the comics.
Cosmic Boy made a return visit later on to give Clark a Legion ring he could use to send Doomsday to the future so the Legion could kill him. Lois inadvertently used it instead.
Honorable Mention: Booster Gold
As of this writing, they haven't even cast the part yet, but I had to put him in the list simply because he's one of my favorite heroes and the chance to see him in action before the series closes down shop was a nice treat from Geoff Johns.
I sincerely doubt we'll see the blue-and-gold spandex (or the goggles). I'm sure we'll get something similar though. I'm interested to see if Johns plays off the Legion angle and brings them in to catch him since he's apparently considered a thief in the future for stealing the Legion flight ring and the technology.
UPDATE: In the weeks since I wrote this post (I try to stay about 2 months ahead when possible) they've given us an image of Booster Gold. He really remains pretty faithful to the character. I can't wait to see the episode and see what they do with him. Here he is:
So that's my list of top guests in the series. I know I missed some folks, but I wasn't too impressed with the Star-Spangled Kid, the Wonder Twins, or Cyborg. Like I said before, I'm going to miss this show. It may have gone down hill after Michael Rosenbaum left the show, yet it was the only outlet we had for DC superheroes in the looooong years between films.