And you all thought I was done with these, didn't you?
As long as the universe remains of a victim of "sequelitis" (and if you need proof of that, look at the Ratchet and Clank series), I will be there to point out the missteps and failures along the way. That having been said, let's take a look at where it all started...
Worms (PC, Amiga, Commodore 64, Game Boy, Game Boy Color, Game Boy Advance, Genesis, Dreamcast, SNES, GC, PS1, PS2, Saturn, XBOX, 1994)
Now THAT is a long list of game consoles. Originally intended to be more realistic (complete with blood, gore, and war sounds in the background), the original Worms had a lot of things going for it -- an interesting war command sim, unique weapons, and a sense of humor. Essentially, it was a violent use of physics (which is basically what Crackdown and Psy-Ops were, anyways).
You could name your worms (giving sheer joy to idiots everywhere), and the environments were entirely destructible, giving an interesting twist to the genre. Speaking of unusual twists, the game featured support for four players...back before anyone knew the internet existed. So really, it was a matter of pushing the keyboard over to your friend. Still, this would be a landmark for the series.
Really, the only noticeable problem with Worms was playing solo. Either the computer was retarded, or it never messed up. Honestly, you can only get whomped on so much by a fake person.
Worms 2/Armageddon (PC...etc, 1997)
See, for me, this is the pinnacle of the Worms series. I include Armageddon with Worms 2, because basically, Armageddon is just an add-on. There really isn't a huge difference between the two.
Worms 2 took everything that was good about Worms and made it better. The sense of humor returned in the form of outlandish weapons (using a Sheep, Skunk, Mole...etc as an explosive instrument of war), and zany character animations.
Also, the improved graphics made it possible for players to create their own levels -- a huge plus. In addition to revamping the graphics and gameplay, Team 17 also went and implemented WormNET, an online place to play the game. Likewise, for people without internet, there was a "Mission Mode" that made playing the game one-player more challenging, and also allowed the player to unlock weapons (like the Aqua-Sheep).
Really, I can't think of anything that could have been done to make this game any better. Any problem from the first game was taken into consideration, and addressed accordingly. Most people would agree with me, but a lot of people think that games are always better in 3D...
Worms 3D/Worms Forts: Under Siege/Worms 4: Mayhem (PC, XBOX, PS2, 2003)
Damn it Team 17, if it ain't broke, DON'T FIX IT. I can't fault them for trying, though.
The 3D forays were not largely well-received, and they earned it: if it's difficult to aim a bazooka using wind on a 2-D plane, imagine how hard it is to do in 3D. Needless to say, lot of frustration there. Also, the wonky camera and confusing physics didn't make things much better.
Again, Team 17 experiments. Some ideas are good (Forts was a pretty interesting spinoff idea), and others are terrible (platforms in 3D). Between the two, Worms 4 came out, and didn't really impress anybody. By the time the game turned out to be playable, it was already too late.
Realizing that the series hit its optimal point with Armageddon, they decided to work backwards. Recent releases, such as Worms: Open Warfare (PSP, DS), and Worms (XBOX Live), are basically just ports of Armageddon, except modified to be used without a mouse (with mixed results in controls...I still can't dominate the ninja rope without a keyboard).
As a matter of fact, those games are all available in some form: Don't just take my word for it -- download!