I may be alone here, but here's my recommendation:
AT&T may be the best option for a smartphone because their data plan gives speeds in the range of 1000-4000kbps on 3G (average about 2000kbps) and 2000-8000kbps (average about 3000kbps) on HSPA+. If Verizon or Sprint don't offer 4G in your area (they only offer it in a small number of cities), then their phones use EVDO 3G, which has an average speed of about 700kbps (for Sprint) or 1100kbps (for verizon). This means using the Internet and watching video on a Verizon/Sprint phone will feel slow, and video quality will either be lower, or you'll buffer your videos more. Furthermore, WiFi hotspot functionality is MUCH more useful on ATT 3G then VZW/Sprint 3G; for one, AT&T lets your phone make calls while hotspotting, and secondly, AT&T's hotspot speeds (usually around 3000kbps), caps (4GB) and prices ($20/month surcharge; also gets you an additional 2GB data to use for both tethering & phone) are all better than the competition.
AT&T also plans by end of this year to transition their network to a full HSPA+ 42mbps network, whereas Verizon and Sprint don't have plans for a full transition away from EVDO till around 2013. When AT&T transitions to a HSPA+ 42mbps network throughout, you should see 5000-10000kbps speeds on your phones.
I have an AT&T phone, and i've also used Verizon service. My AT&T phone speedtests usually in the 2400kbps range with 5 bars, despite not being a HSPA+ phone. If i have a 2 bar somewhat weak signal, i get about 1500kbps. A HSPA+ phone i tested got about 3200kbps in the same area. Verizon phones i've tested get about 900-1200kbps and Sprint about 600-900kbps with 5 bars. All EVDO phone drop to about 600-800kbps with a weak signal.
The net result is if 4G service by Verizon or Sprint isnt in your area, you'll have difficulty watching video, downloading apps, playing online games, downloading video or audio clips, and using WiFi HotSpot function on your Verizon or Sprint phone. Meanwhile, browsing will also be somewhat slower.
What I would recommend is getting the Atrix 4G or Infuse 4G from AT&T. These phones can easily pull 2-4mbps, and the Atrix in particular has a dual core processor which when combined with AT&T service makes loading web pages much quicker on 3G than any other 3G network/phone combo out there.