Easily Joshua Tree
The opening 3 songs of JT is their finest run of 3 ever and is what hooked me on the band. Basic take, I know, but it's basic for a reason. Bullet, though better live, is a classic. Running to Stand Still is incredible, one of their best.
I like all of the first 5 songs on UF quite a bit, but they're not even close overall. Don't come back to Promenade or Wire very much. I adore A Sort of Homecoming, and Pride is a monster classic of course, but they don't measure up to those first 3 on Joshua Tree either.