What network iPhone should I get?

I'm looking on US ebay for an iPhone to use here in Ireland. What network would be best to get, what network can be unlocked here in Ireland and used here with my o2 sim card?

ATT and Tmobile (GSM iphones)

do NOT get Verizon and Sprint and Virgin Mobile and Cricket and MetroPCS iphones… They are CDMA, the ESN is most likely bad, and can't be unlcoked by any hack to work overseas since they are CDMA and not GSM…

Unlocked GSM phones would be your best bet. You get more options for service.