It depends. The Mitsubishi 4G63T mentioned by the parent was pretty solid. Even most of the aftermarket turbos sold for it were reliable too. I've had good experiences with Volvo turbos too.
For lighter aluminum-block turbo engines, I'm a fan of the 4B11T (in my daily driver, an Evo X) and the BMW B58 (with continued development, I think it will become the greatest inline-6 of our time, this generation's 2JZ).