It varies by phone. I've used a lot of phones and the iPhone was toward the weak end (not as weak as some cheap Samsungs, though). My Nexus One had poor 3G signal, while most of my Nokias and Sony Ericssons have strong signal.
These findings are sometimes difficult to come to if you live in a big city with a lot of cell towers/antennas, but if you live in a rural area (as I do) where you sometimes rely on just one or two cell sites, it's easy to notice what phone is weak.
Past providers: Page Plus, AT&T, T-Mobile, Virgin, Straight Talk
Past phones: Nokia 820, Motorola Atrix HD, iPhone 4S, HTC One V, iPhone 4, Nokia Lumia 900, Nokia N9, Samsung Focus, Samsung Captivate, Nokia N900, HTC 8525, iPhone 3GS, iPhone, Nokia E61i, Siemens A56...