The 4.3" display models like the HD2 and HTC Evo are obviously the widest and thus the hardest to one-hand. But if you have quite large hands, you might have few problems. I have large hands for a woman with very long fingers and I use the HD2 one-handed without a problem. I lay the phone in my palm across the fingers and my thumb can reach anywhere on the display.
It is true that the market is moving toward multi-touch and away from one-handed operation. Among Windows Mobile HTC Sense phones (generally my top picks), they're all on the large side.