Here's some more info I decided to type and some pics to illustrate.
In Irwin series 1, figures weren't painted in areas that could be achieved with colored plastic. The paint they used to match unpainted plastic was like an exact match, very well done. Raditz in my earlier example is from this series.
Series 2 was more of the same but they didn't use as glossy of plastic on skin as they did on Nappa and Raditz in series 1.
Series 3 was more of the same, no glossy skins, which was nice. Glossy pants on Future Trunks... not to nice IMO.
In Irwin series 4, see my post above for pics, they experimented with glossy plastics and matte plastics, and everything was pretty much good either way. There's a lot of loose production samples floating around in people's collections, most of which came out of Irwin's bankrupcy and production rejects from China and some of these look like crap, especially the Piccolos, because his pant legs won't match his crotch color.
In series 5, Irwin tried out a different style of doing their figures... which freaking sucked. Unpainted blue plastic, unpainted yellow plastic, and unpainted orange plastic with some sort of disappointing texture to it combined with horrifying dead looking single colored flesh colored paint. Series 5 overall sucked as far as paint jobs were concerned on every figure in the set. Pitiful.
In Irwin's 6th series, they started their fancy paint jobs. They all looked freakin' cool to me, except they shaded most Android 19s too much and the speckles on the SS hair suck.
The EB Games (a store that doesn't exist anymore) exclusive didn't have the speckles apparently... I didn't know that until now... in fact, I didn't know this specific existed until I looked at the checklist on DBF... Does anybody here actually own this to verify if there are any other differences? I only thought there was a gold haired non-SS Trunks and a gold haired SS Vegeta until today.
Anyway, series 7 came next, and you can see that Goku in my earlier post. The figures, in my opinion, ended up painted significantly better. The Great Saiyaman, SS Gohan in blue gi, Majin Vegeta, and Spopovich are painted reaaaaaally amazingly IMO. I know some people don't like that Gohan isn't painted wearing dark blue and red, but I actually love Irwin's unexplained variation of medium blue and salmon-orange.
After that was series 8, and this was probably Irwin's best painted series. This Angel Goku is epic. Daburah, SS Gohan in black jumpsuit, SS Goten, SS Trunks, and Videl... all epic.
Series 9 was more of the same awesomeness. My only complaint is that Hercule's eyes were always muddy looking.
Series 10 was a little weird. Yamcha and SS Vegeta with Halo didn't end up with any/barely any skin shading at all. They still look good, but it is weird. Gotenks was done just like the previous series, nice shading. "SS Gohan" who we all nicknamed School Suit Gohan has horrifying dried-blood colored skin shading. Even though I'm only showing Goku pics, it's worth showing his gross skin. This one isn't as bad as the one I bought brand new. The head of this figure is not as good as the prototype Irwin showed, so it's weird. Yakon in this set is freaking amazing, much better than the Jakks rerelease that had purple shading.
Series 11 was simiular to 10, but Irwin filed for bankruptcy shortly after this series was released, or perhaps before it was released, I don't recall. Kai Suit Gohan lacked much skin shading, and was painted in the wrong color clothes for some reason. Kibito and Kaioshin were amazing other than eyes with too thick of black outlines. Bulma strangely had didn't have painted skin, but she still looked great, particularly if you found one with the eyes painted in the exactly right position. Skinny Buu was awesome. SS3 Goku kind of sucked. Barely any shading on his skin, an ugly face, weird hair. For those that don't know, he was rare as heck when first released. It took me months to get one, as none of the cases my local comic book store received had him in them. A few eventually made it onto ebay, but their prices were $50+. After Jakks announced they were taking over the DBZ license, we started seeing this type of packaging
Instead of this type we were used to:
And SS3 Goku was no longer hard to find, particularly in that new packaging. There's no difference between any of the figures as far as I know between the different packaging.
So that was the end of Irwin and the start of Jakks.
Jakks picked up where Irwin left off. Series 12 was their first. The figures were no longer painted and then shaded. Instead they were usually colored plastic when possible (painted when not) with airbrushed shading. These all looked great as far as paint and everything goes. The SS Gotenks looked like he was rushed or something. He has a weird face sculpt. The mystic Gohan was for some reason really tall, but since Irwin made such a tall SS Gohan in series 7, we weren't all that surprised.
Series 13 was another great series done just like the last one. The only bothersome things were that Goku grew about a foot in height for some reason compared to Irwin's Gokus, and the face sculpt of SS Vegito was ughhhh. The new paint/dye whatever process still looked great.
Series 14 was more of the same. There were some things to not like about Vegeta and Trunks, but nothing related to the paint. This Goku has the coolest arms Jakks ever made and my favorite for customs.
Series 15 is when people started getting pissed off at Jakks. They rereleased SS3 Gotenks from series 13 with cool looking pearl yellow hair, but horrible looking battle damage. The rereleased Irwin's SS Goku in armor with great hair but horrible looking battle damage. They rereleased Series 13's battle damaged Goku with a series 12 Gohan head. The good stuff? They gave us a cool Namek saga Vegeta (with the wrong color of blue outfit, but everything else was cool) and one of Jakks' best figures! Cyborg Frieza! Strangely with mystic Gohan's sword lol.
Somewhere around this time they were doing 2-packs and other series besides the regular releases and I'm not sure where everything falls into place.
Series 16 is officially the first series I could no longer find. Walmart, Target, CVS, Eckerd, none of them had DBZ figures anymore. Only Toys R Us carried them. USS Trunks was cool, shirtless Piccolo was cool, this SS Goku was cool other than being too dark skinned, and they released an SS Gogeta in this line for some reason... and gosh it was ugly.
Series 17 was basically annoying, as Jakks was just rereleasing stuff and making stuff up. They released this Kaioken Goku with unfortunately the Art of Fusion big SS Goku head, arthritic hands, and no wrist bands. The rereleased Namek saga Vegeta, but with SS hair. Striking Z Fighters Recoome wearing darker navy blue clothing (an improvement), Irwin's Burter with matte paint instead of glossy, and Striking Z Fighter Captain Ginyu... and I don't know if he was different or not.
Series 18 was more rereleases, mostly weird stuff. I think it was released in 2004. A transparent green Piccolo, pearl pink kid Buu, Energy Blaster SS Trunks with purple jacket, Cyborg Frieza looking no different (as far as I know), Irwin series 2 Frieza form 2 with transparent arms and legs, and a shirtless Goku with blue cloth shirt. Basically just weird stuff.
Here's a bunch of random stuff I can't fit in chronologically without a bunch of effort and knowledge I don't have...
The Art of Fusion SS Goku on the right was the first to have this body they reused over and over. The head was huge, okay looking, but so much larger than any other Goku head that it was like a bobble head.
At practically the same time, Jakks released a much different Art of Fusion two pack and the SS Goku in this was an absolutely beautiful figure, probably Jakks' best SS Goku ever. Way better than Irwin's.
This was in an early two pack with Raditz and it was good.
I don't know the chronology after this, as I stopped collecting and couldn't find the figures anymore, and the figures were almost all minor tweaks and crap I didn't want so I didn't care. For over a year Jakks did this stuff and quality went up and down.
This weird looking Goku with barely any neck and a brown face (wish I could find a pic where the seller used the camera's flash!!) but a nice symbol on his shirt was released in the Alien Invaders series... major quality suck IMO. And this was released with the likes of the awesome King Cold, and the Arlian Nappa and Vegeta!
Here's a bunch of random Gokus.
Those damn arthritic hands ended up on so many of them.... They were probably trying to make them look like Kamehameha hands, but the fingers just look tense and crippled.