|Lina and Gourry
These two are so perfect for each other! Just watch the way they interact and it's obvious they fit. All the Slayers couples are based on contrast with a few similarities. These two both have common interests like traveling and eating like maniacs, so that their lifestyles merge easily. But their true strength comes from their differences which is what makes them such a strong team.
Lina is a very dominant individual and likes to be the one making all the decisions. Gourry is very easy going and so he is perfectly willing to let Lina decide while he follows along. Lina's intelligence helps keep them alive and ahead of the bandits, while Gourry's instincts sometimes pick up what she misses (I'm not really sure if he's as stupid as he acts or just lazy when it comes to mental exertion). Lina's personality can sometimes get out of control when she lets her greed or anger get the better of her, and it is those times when Gourry can provide a moderating force that helps persuade her to do the right thing. Lina doesn't really need an intelligent companion with whom she would be inclined to argue, as much as she needs the absolute loyalty and acceptance that Gourry gives her. It is rare indeed to fins a man who would be willing to put up with the abuse Lina likes to dole out. He occasionally whines a bit, but he seems to understand her need for stress relief and he had an amazing ability to take whatever she gives him without too much harm. And Lina may abuse him, but when he really needs her help she'll be there for him, even giving up the entire world for him.
Zelgadis and Amelia
This couple is one of the most contested ones among the fan community in spite of the fact there is almost as much evidence for them as for Lina/Gourry. They also demonstrate a couple who are perfect compliments. Zelgadis has an extremely low self confidence and can fall easily into depression. He has had a very difficult past and has seen far too much of the darker side of life. Amelia, on the other hand, is innocent and still believes firmly that Justice Will Win. Her cheerfulness can help Zelgadis see the good in the world. When he looks down on himself she is there to tell him that he doesn't need to change, that he is worthwhile even without a cure, and that she'll love him no matter what. Her bubbly personality may get excessive at times, but that's when Zel is needed to bring her back to reality and keep her from getting herself killed. Zelgadis is also rather stubborn and set in his ways. He needs someone who is willing to sacrifice, and to accept him the way he is rather than insisting he mindlessly follow like Lina does. But he has a lot to offer in return, and he shows, by his willingness to place himself between Amelia and whatever danger is around, that he deserves her too.