The Slayers is one of the few anime I'll ever give a 10 out of 10 rating to, and there's a reason it's my number one all-time favorite anime. Though a lot of it is generic fantasy and spellcasting, the characters bring the story to life.
The anime itself was adapted from a light novel, and though it doesn't follow it to the letter, it's still interesting and entertaining.
I will also say that this is one of the few anime where the English dub is just as good as the original Japanese cast. I have seen both versions, and the voice actors really bring the characters to life.
Story
There are two major arcs in this season. Both are fairly generic plot-wise for a fantasy story, though there are slight twists to them.
Because the plot is slightly generic, I did dock points, but it gives off a tabletop RPG feel (in fact, that's what brought about the idea for the series). However, that sort of plot makes it easy to dive right into the world and not be overwhelmed by terms, locations, and various species.
The Rezo arc starts when the protagonist steals an item which is worth more to sorcerers than originally thought. One of the great sages, Rezo the Red Priest, gets involved with the item and the protagonists, and there's something even darker behind his goal.
The Copy arc contains a lot of the filler episodes, which have their own merit. Someone has put out wanted posters for the protagonist and her party, which makes the protagonist seek out the person who did it. A mercenary and a strange sorcerer with a red gem on his forehead show up to clash with them, and these meetings lead down the road of something much more dangerous.
Characters
If you're watching the Slayers, you're most likely watching it for its characters. I don't think my descriptions can give them justice; they're just that charismatic.
Lina Inverse isn't your typical teenaged protagonist; nope, she's a sorceress who falls into the chaotic good side of the scale, stealing from bandits and giving to...herself. She's a sorcerary genius with a petite frame she's sensitive about, and a temper hotter than her fireballs.
Gourry Gabriev is an extremely talented swordsman with a very unique weapon, which makes up for his absolute lack of braincells. Often referred to as "jellyfish brains" by Lina, he's not the sharpest tool in the shed, but is fiercely loyal and protective of Lina.
Zelgadis Greywords is a mysterious sorcerer who has a variety of shamanistic spells in his arsenal. His appearance is startling at first, as he's part rock golem, part brau demon, and part human; magically altered from being 100% human.
Amelia Wil Tesla Seyruun is a girl a little younger (but more developed) than Lina, who appears in the second arc and specializes in healing magic and astral attack spells...along with giving long winded speeches about the "hammer of justice". Despite her good nature, she's perhaps just as hot-tempered as Lina.
Sylphiel Nels Lahda is another female introduced in the second arc, and has an even greater ability with healing magic than even Amelia. She's shy yet very down to earth, and has had a crush on Gourry since they met before the series started.
Overall
With excellent opening and ending songs, and fight scenes/animation quality which are considered at the top for a 90's anime, along with a cast of colorful characters who develop nicely and comedy in just the right places...it's a must-watch for all comedy-fantasy fans.
The pacing is excellent, and even the filler episodes don't feel like fillers because of the sheer hilarity they introduce, as well as bringing to light some other sides of our beloved characters we might not have gotten to see in the story itself.
I believe all plot points brought up in the story are completed and resolved within the arcs; I don't recall questioning anything for plot holes.
I have shown this anime to everyone ever since I was shown it, and have even gotten a few non-anime fans hooked on it. It's not well known, it has never aired on TV in the USA, but it deserves much more credit than it's often given.
Certainly, it deserves a full 10 out of 10 rating.