I strongly recommend Abus U-locks, they can't compete on price, but they are extremely reliable and have the best mounting bracket on the market. Their square keys (from the Abus Varedo 47 model and up) are also extremely strong, unlike some keys from cheap u-locks that seems to break far to often.
My oldest Abus U-lock (a Junior) is perhaps 10-12 years old. I gave it to a friend 5 years ago, and is has practically lived outside in the rain and snow ever since. It still works. My present Abus Varedo 47 is perhaps 8 years old. I lock and unlock it 2-3 times a day, so it has probably endured around +15000 locks and unlocks over the years, and it still performs perfectly.
While I usually place the bike indoors when it rains, I live in a climate with year round high humidity and near the sea, so everything tends to rust very fast, but so far both Abus locks have been fine. I do however give them a spray of light oil every 1-3 years to protect them.
The bracket has developed some play over the years, but it still holds the u-lock firmly in place and it doesn't rattle, so I don't bother replacing it. I just love how easy it is to use the bracket (a TexKF Twin style bracket); the locks is removed or attached in seconds with a very natural movement.
An Abus "Varedo 47" with a "TexKF Twin" bracket is a very good choice, it is strong enough that it requires motorized tools to cut, while not being too heavy. The Abus "Granit Futura 64" is more secure, lighter, but also more expensive.