When it came to fight, I think that both Spike and Angel are performing in team with Buffy, but at different levels.
But my answer will be Spike for a very simple reason (and I swear, I try to put aside my shipper bias 😛 ). We have seen that their team works when it came to fight together, side by side, like in The Killer in me, when they instinctively put themselves back to back. We know that they wouldn't hesitate to trust each other with their life, and even if we don't have see them in an effective battle a lot of times, the fact is : he's there for her, in the middle of action, constantly. It can be on little details, but look at Wrecked, when they rescue Dawn from the demon who's attacking her. They really act as a team. They both heard Dawn screaming, they exchange one look and here we go, they're here for rescuing her. Buffy is fighting the demon while Spike is looking after her sister. To me, working as a team is knowing how to be additional. In Dirty Girls, it's Spike who saved Xander from Caleb, and Buffy comes to them just after that for helping. When one of them isn't there, the other one takes the role. We also have the example of The Gift, when Buffy is fighting Glory, she asked to Spike to going searching Dawn at the top of the tower.
Maybe I have a wrong perception of the situation, but I always felt that Angel was both near and away from her. He was mostly alone, while Buffy was fighting by herself (or with Faith in season 3).
And we see Spike and Buffy fighting side by side a few times, in Listening to fear, Flooded, All the way, Tabula Rasa (they instinctively form a team), Dead things, Older and far away, Help (he helped with Cassie), First date, Storyteller (and "Spike" is the first thing she calls for when a guy caught her), Dirty Girls, Chosen... And even when he doesn't fight, he's there, like in Spiral, where he saved her life, or in OMWF where he did it again... At the beggining of Dirty Girls (just before Faith's arrival), we know they're patrolling together , and it doesn't seem to be the first time. And during season 7, they're coaching the potential slayers together. They're not only a team when it came to fight, but at many different levels, and that's an important part in the reason why their team works.
But honestly, that's not exactly why Spike has my vote. I think their team is more effective simply because of their emotionnal connection. I mean, if Buffy hadn't believed in him, like she said to him in Never leave me, he probably wouldn't have keep faith in himself. But she supported him, protected/defended him and trusted him, and at the end of the day ? He saved the world. And vice-versa. Buffy was completely broken in Touched and had lost her faith in herself, but it's Spike who give her strenght to fight and to find herself again, by connecting her not only to him, but also to herself. And finally ? She kicks Caleb's ass, and found the Scythe who permitted to give power to the potential slayers. That kind of connection is what makes them stronger in my opinion, because they're additional and they're able to give strenght to each other, because of their emotional connection, and that's precisely this kind of strenght who permitted to save the world. By this point, we see a team where the relationship between the characters is positive for them, but at the end of the day, also positive for the world.
So yeah, undoubtedly, Buffy & Spike were the best team in my opinion.