Spike was the best thing to happen to Angel season 5.
He brought so much to the show in terms of conflict, humour, and also helped it get back to the shanshu prophecy and Wolfram and Hart which had been sidelined, and kind of brought those stories to a head too.
He and Angel have one of the best relationships of any two characters in the franchise. There was obviously a lot of potential for humour like Astronauts vs Cavemen, whilst there was just as much potential for really great drama like in Destiny. They could switch from Frasier and Niles as Vampires, to the bitterest enemies in an instant.
At the same time however it wasn't all just conflict. It's subtle, but as the season goes on you can see that Angel and Spike develop a special bond. They can share things with each other that they can't with anyone else, they can relate to one another better than they can with anyone else, and surprisingly they trust each other completely when fighting together. Also rather touchingly Spike does genuinely believe in Angel, whilst Angel similarly calls Spike a hero. I love Spike's reaction to hearing Angel called him a hero, and Spike being the first to join him in the final fight.
It's funny because in a way Spike feels more at home with Angel's gang than Buffys. With the Scoobies everybody hated him, but with Angel's group he not only has Angel, who underneath it all knows what he is going through, but the rest of the gang, who having been through it with Angel are at least a bit more sympathetic to Spike.
Season 5 did have a lot going for it in the sense of it had finally dumped the awful Connor story arc and Cangel (though Cordelia herself was a big loss and Charisma's departure should have definitely been handled better.) Still I think it needed something else to really help it get over the disaster of season 4, and Spike definitely did that.