Scala's. You can't get any closer to Union Square than the St. Francis Drake Hotel where it's in (with the doormen in beefeater costumes). Scala's is my "safe place" to take visitors when I want a solid meal but they don't want to leave all the shopping. It's the compromise I offer when I want to travel across town to a hole in the wall and they keep gazing over at the prominently placed Cheesecake Factory atop Macy's.
It's white tablecloth and bustling but not noisy, so it should bve nice for a business dinner. Really nice interior, lovely service, and basic Italian menu items that won't scare anyone. However, some of the pastas (homemade ravioli changes daily) and the fish dishes are quite nicely prepared so you won't have to sacrifice quality.
Cocktails are great, and I hear so are desserts. It's not the best SF has to offer, it's not innovative, but it's consistent, convenient, and satisfying. A little pricey, but it's Union Square.
For even more basic food, Daily Grill has good prime rib and more meaty American classics.