My favorites were always Death Star Vader (because it was actually pretty cool... but also had a pain in the ass transformation), and Yoda because it was so fucking stupid it was good.
I enjoy the Millennium Falcon that splits into Han and Chewie. As for worst, never get either of the Luke Skywalkers (X-wing or snowspeeder). The alt modes are ok, but the robot modes are awful, especially the Snowspeeder one.
I liked the smaller Cyberverse scale figures...the tend to have more reasonable proportions in robot mode than a lot of the larger scale figures. But they were insanely overpriced until they started showing up at Five Below. And, unfortunately, they only made five characters (three different versions of Anakin and two of Obi-wan), with only one villain (Grievous).
My personal favorites are the Magna Guard Droid/Cad Bane, AT-AT, TIE Bomber, and the Republic Gunship. The Y-Wing's not too bad, I guess. I've owned almost every figure in the line. A lot of them aren't that great, but might have superb alt modes. If you're a Star Wars vehicle collector, you wont be disappointed.
The only one that ever looked cool enough to me to bother picking up was the Darth Vader Death Star. It did not disappoint. At ALL. I have him displayed next to my Unicron and Primus, and he fits right in. Seriously, this guy has SO many accessories, so many gimmicks, and all of them are actually really clever and interesting. Death Star mode is entirely perfect (and even has flip-out stands so you can display it), and Darth Vader mode is huge and imposing (as a planet-former should be). A bit hard to come by complete without buying it in the box, but entirely worth it, in my book.
Quality control was horrid on mine and all, don't know if that was a regular thing or just mine, but thought it was worth mentioning. Both leg balljoints (or somewhere round there haven't had a look in a year) snapped off one of them just looking at it. I always found the Falcon and the Death Vader to be the best, although I didn't buy many more Luke left a bad taste in my mouth.
My favorite is the Han and Chewie Falcon. Death Star Vader a close second. And I LOVE Yoda for the sheer goof factor. And while it has it's faults, the Storm Trooper Walker is pretty cool.
You forgot to mention the most important accessory, his cloth cape! Definitely the best of the bunch in my opinion.
God yes, the cape. That falls under my "accessories" comment, but it's definitely worth mentioning for the sake of it. Have there every been ANY other Transformers with a cloth cape? I know there's stuff like DOTM Megatron's cloak, or Cybertron Vector Prime's fake-cape, but are there any other legitimate capes in Transformers? I don't think so.
For the larger figures, generally speaking, the not-named characters were better. I really like (and still own) the TIE Bomber, Magnaguard, and Y-Wing; of the three, I'd say the Magnaguard mold is the best. The Cyberverse-sized figures were moderately okay, and I do lament that the line only got three molds. Of the two I own, I far and away like the Delta starfighter over the Eta.
In fiction there are several instances. Most notably Starscream's coronation cape was probably made of cloth in the movie, but the MP toy's cape was made of vinyl I think. Toy-wise I'm 99% certain that Vader is the only Transformers toy with a cloth cape.
As cool as the Millennium Falcon pair was, it was such a fucking stupid idea to use gold plastic. Yes, my Han and Chewie broke due to GPS, and ALL Millennium Falcon TF releases have it. My favorite would have to be Death Star Vader. So badass.
You do know the legs extend yes ? ---- I rather like Cade Bane I also like the Turbo Tank, The V-Wing, and in general the ones with clone trooper alt modes - I'd suggest at least 1 Jedi (delta) Starfighter too such as Anakin or Kitt Fisto.. but in general Alien faces work better than Human ones. The Tie Bomber is not to bad either - pretty decent robot mode too. I'd love Yoda and a Republic Fighter tank but neither ever came out in the UK - unfortunately. I was also unable to get the large Y-wing / clone Pilot but I have the anakin version and that is OK (I suspect the 'matchstick' version is much better though. It might be easier to say which ones to avoid than which ones to buy. but remember some of the earlier ones are best bought in the 1st release version so you get the mini-figure. I'd have like the Grevious figures more if the'd had 4 arms as an option. Darth Maul isn't terribly brilliant either - but might be worth getting if only as the line is short of villains.
I love the Han/Chewbacca Millenium Falcon. I have it in storage, too scared to take it out because apparently it can have GPS.
Best leave it in robot mode then. They were quite common at Yard sales - perhaps I better get a knackered one for spare parts then.