It's an oversimplification but basically T-Mobile.

The cheapest monthly plan you can get through T-Mobile is $29.99 which gets you 300 whenever minute, unlimited free weekend minutes, and free long distance and roaming. Sprint's Basic 200 plan is nearly as good, while it only gives you 200 anytime minutes, it also gives free Sprint roaming, unlimited free weekends and adds unlimited free minutes after 9pm. 

Meanwhile the cheapest plans AT&T and Verizon Wireless will give you are both $39.99 and for 450 minutes. On AT&T those minutes roll over if not used, and they also throw in 5,000 night and weekend minutes, unlimited mobile to mobile calling. along with free long distance and roaming on the AT&T network. Verizon counters with unlimited nights and weekends, free IN calling minutes, and free long distance as long as it's on the network. 

Alltel also has a basic plan costing $39.99, but it's for 500 minutes. That "National Freedom" plan includes free mobile to mobile, free long distance, and free nights and weekends.  

Still if you're just looking for an individual plan and looking for the cheapest plan possible I'd say T-Mobile gives you the best buy. 

Via here and here