OK, so after a recent essay about this was made (which was very well thought out), I decided to make my own based on my perspective. Here goes.
First Era: 1986-1993
This was the early era and arguably the "classic" one. It gave us what are probably the strongest books of the series, with tons of classic characters, ranging from Matthias, Cornflower Fieldmouse, Basil Stag Hare, and Queen Warbeak, to Gonff the Mousethief, Chibb, and of course, Martin the Warrior. The ideas were fresh and new at this point, and each book serves a bit of a purpose.
Redwall introduces us to the world of the series and sets the bar for the rest. With Mossflower the series truly settles into itself, and tells us the legend behind the first book, which is a fantastic idea. Mattimeo serves as a direct sequel and brings back a villian from the first book. Mariel introduces us to the Abbey in a time before Matthias's, gives us our first female heroine, and also gives some backstory on the bell that killed Cluny. Salamandastron gives us our first non-mouse protagonist (Samkim) and is also a little darker in some ways than the previous book---a reminder that this series isn't for little kids, and in some ways as a lead-in to the next book. Martin The Warrior wraps things up by giving us another story about Martin the Warrior, but this one having an unusually dark and bitter ending...one that arguably makes it the best book of the series.
All of these books are classics and are Redwall in it's prime. They're also united by the fact that they were all illustrated by Gary Chalk.
Second Era: 1994-2000
This was the era in which we start to see some decline in quality of the series, in my opinion. They could still be good and give us great characters, but they were losing the freshness.
The Bellmaker started things off with the second (and last) direct sequel to a previous book, this time to Mariel of Redwall. Whereas Mattimeo had been a great, if not better sequel to Redwall, this one was just sort of meh, in my opinion. The book just felt pretty dull. One character who used to be "strong and silent" inexplicably turned into a wimp, a few characters from Mariel were gone or didn't get a large part, the new characters (aside from the villian), were forgettable in my opinion, and the story just felt formulaic. Oh, and did I mention it started the ever-increasing trend of obnoxious dibbuns? Before this, we had Baby Rollo and Baby Dumble, who were tolerable, but here we got that Mousebabe that Mr. Jacques couldn't even bother to give a name, and whom did little except be cute. It was also here that the term "dibbuns" started to be used commonly.
Anyways, I've gone on enough about that book. What else was wrong? Well, the series as a whole was still fairly creative, but starting to lose it's flair. There are a few books in this series I haven't read in their entirety but know enough about to give my thoughts on this era as a whole---it was good, but it didn't quite compare to what came before.
Probably the strongest books of this era were Pearls of Lutra (the one which introduced Martin II, who was nothing but a shell of his namesake), Marlfox, and Outcast of Redwall, quite possibly the most controversial book of the entire series. Why? Because it promised a truly thoughtful storyline about a ferret who must choose between good and evil, not the story of Sunflash the Mace, who, admittedly, was due to have his story told, having been introduced six books earlier in Mossflower. Said promised storyline ended up being a complete letdown in many readers' eyes, as it basically suggested that all "vermin" creatures are born evil, as well as the fact that even the one character who believed there was good in the outcast ended up agreeing he was evil after his redeeming moment of saving her life.
OK, I've made most of this section into a rant about The Bellmaker and Outcast and I'm sorry. Let me try focus on the good stuff.
This era continued to bring back characters at times, and it still had prequels. There were far-off lands visited in Pearls of Lutra and Marlfox, which is very awesome. (The kingdom in the south in The Bellmaker wasn't as much, in my opinion.) We still got some great characters, and some great riddles, espicially in Pearls of Lutra. We got to see glimpses of the world of Redwall through different illustrators with Allan Curless and Chris Baker. It still felt like Redwall, just that maybe Mr. Jacques was saving up better things for later. Sadly, this was not the case.
Third Era: 2001-2008
This was unquestionably the weakest era of the series. I haven't read many books from it but know enough about it to know how poor it was. This was when the series became particularly formulaic: each book introduced a new cast with practically no connection between any of them. The basics were all there---the questing, the riddles, the food descriptions---but it was all formula now, just the basics of what once was. I believe it was around here that Jacques increased his use more ridiculous names for characters instead of real or descriptive ones, and dibbuns were more obnoxiously used than ever, from what I can tell. There were a few redeeming things, of course: for example, Triss brought back Brockhall and gave us a three-headed serpent, practically, and we got to see the Redwall world again from the perspective of two new illustrators. But there was no question about it at this point---a once great series had declined into a cookie-cutter franchise, lacking almost any of the magic it had once had.
Fourth Era: 2010-2011
The last two books of the series were, from the sounds of it, the start of a new era. I haven't read The Sable Quaen, but apparently it was very good. It featured a mole warrior for once, and brought back the Flitchaye---signs that Jacques might be planning to be more creative while revisiting "classic-era" stuff.
Sadly, only two books into this era, Mr. Jacques tragically passed away of a heart attack, and what lay ahead for the series remains a mystery. I really like to believe that Mr. Jacques would have fully gotten out of the creative slump he had been in for so long (and possibly not even realized), but it's possible that we would have simply gotten another decade of cookie-cutter books, with Jacques eventually spinning it off into a series of picture books (a la, A Redwall Winter's Tale) as age started to catch up with him and full-length novels started to take longer.
Whichever way, we still would have had the three main eras we have here.
So what do you think? Leave comments below.