The HP is a really nice screen. It has wide viewing angles (IPS) and is super fast (almost no lag). I'm not sure whether the Dell is the same at the lag-aspect (same panel does not mean same electronics). The NEC is different, it's a 21.3 inch PVA screen. Bigger and better blacks, but a lot slower (2-3 frames input lag).
Don't go for the Happ. It is not very clear about the panel used, but it feels like TN (angles for IPS/PVA are mostly 88/88/88/88 or 178/178, not around 80/160). Another thing is the aspect of the Happs, it is 4:5. Sure it does offer CGA input, but unless you have a real PCB, this is useless for PC use. Any modern videocard has way more scaling options than built in CGA scalers.