NEC EA231WMi. It's an e-IPS panel, which is a bit worse than a true H-IPS or S-IPS panel, but it's far better than cheap TN panel monitors. IPS panels have better color accuracy and viewing angles than TN panels, while TN panels have better response time (focused on gaming), but horrible color accuracy and poor viewing angles. Best of all, the EA231WMi can be found for $250-300ish USD.
Dell also has the U2311H which uses the same panel as the EA231WMi from what I've heard. The general concensus seems to be that the NEC is slightly better for a few reasons, though.