almost as many answers to this as there are phones, but for what it's worth my current phone - a Motorola RAZR2 V8 - seems to give me the best signal of any phone I've had so far, and I've had a few, believe me.
The voice quality is just stunning - due, so Motorola tells me, to their 'CrystalTalk' technology.
Frankly, there are so many variables when it comes to signal strengths that it's virtually impossible to be assertive about a particular phone. As far as I'm concerned the Motorola is excellent. It's already travelled around with me quite a bit, and it hasn't let me down yet.