If I only had one day left in Hanoi and only one restaurant/bar from which to choose, I would most certainly enjoy my last meal at Lá. Granted, everyone's tastes are different and what works for me may very well not work for you, but for my money and preferences Lá never disappoints.
Part of the reason I go to Lá so often is because it's one of the first restaurants at which I ate during my first few visits to Hanoi. The atmosphere of the place is not at all like the bar scene from Cheers but (at least to a non-drinker) it seems to offer a wide variety of drinks. It's also not overly formal but to say that the quality of food is always good would be an understatement. I have not once had a problem with the food I've eaten at Lá. It's always prepared well and served with hot bread and butter on the side.
The Vietnamese menu is very good but if you're looking for street food prices or nitty gritty flavor, Lá's not the place to go. The chả cá and bún bò Nam Bộ are excellent but sterile, if that makes any sense. There's a clean taste to the food that lacks blog-worthy "look what I ate in Vietnam" potential but it's very, very good. It's a bit difficult to explain but the best I can say is that if you can only eat at Lá once then opt for some of the other dishes that are sure to please.
My favorite non-Vietnamese food at Lá is the lentil soup, crab cakes, shrimp farfalle with capers, and calamari with yogurt dip on the side. If you've got room after your starter or main then go for ice cream with a brownie for dessert.
All in all this place is my favorite spot in the city because I can always count on the food being good. That's very important to me on those days when all I want is a nice hearty meal and a solid wooden chair in which to sit and eat.