Cricket Wireless has excellent plans and customer services. They provide good phones at significant discounts or for free if you switch to them from another carrier without requiring signing up for a multi-year plan. They have been acquired by AT&T, so their coverage is excellent. You can switch plans online anytime and it will take effect immediately. For example, if you downgrade your plan, you would get credit immediately. This is great for international trips. You can upgrade your plan to one with free international roaming service before a trip, and downgraded it after the trip.
Their customer service is excellent - quick, courtesy and effective.