Wolfram & Hart swatted the First Evil like the irritating little bug it was. No contest which was the bigger threat with a more well-thought-out plan.
The end-all, be-all of the First Evil's incompetent plan was to overrun the planet with a bunch of Hellmouth nasties. All the First managed with its so-important key plan of using Spike was a few dead no-name ladies and Andrew getting a sore neck (and he deserved worse).
Wolfram & Hart sent the whole city of L.A. to another dimension just in retaliation for killing a few key employees and only gave up one city (while having offices still all over Earth) because Angel was too expensive. The Wolf, Ram and Hart remain entirely undefeated in some other dimension and are still completely capable of acting up in the main one. Then there's all those dead employees that still walk, talk and do their jobs forever.
The First Evil was practically getting mocked at the end for being impotent and incorporeal with only the power to taunt. It clearly was getting nowhere in its desire to be corporeal again.
Even Holland Manners alone was better at taunting Angel than the First Evil managed on anyone past Amends (the only time it did anything almost effective). What Wolfram & Hart pulled with Darla's resiring and spiking Angel's drink with Connor's blood was more competent than anything the First Evil managed. Granted, Wolfram & Hart had five years of development on its side, as well.
I'll put my money on the Big Bad that actually defeated the other Big Bad. It certainly wasn't Buffy's dumb little spell that saved the day. She, the Scoobies and all her little girls would be Hellmouth food without Lindsey/Lilah/W&H's shiny bauble and plan to drive Angel crazy with Casper the Friendly Vampire. Buffy wouldn't have won her last battle without Angel being desperate enough to save his son with a little sweetener on the side of saving his ex from her non-existent plan. Without the deus ex machina, it would be Angel fighting a lot of Übervamps with a lot of scared teenage girls with new powers that need to be trained from scratch. He's had plenty scarier foes than them, given that even Dawn could slay them (the one Buffy slayed earlier was clearly the only MVP). The demon horde as a parting gift from Wolfram & Hart he had in the alley was comparable. Jasmine and the Beast were more apocalyptic, seeing as every souled being on the planet was about to become Jasmine's one-minded army. Wolfram & Hart are nigh unbeatable. Only a full-powered Illyria might give Wolfram & Hart a run for its multidimensional existence.