FO 1&2 are one of the best games ever. But they're cRPGS, so if you don't like those type of games you would have a bad time, because it does have a slow start.
FO3 is kinda isolated game. Has the most retarded plot out of the main games.
You can play NV, but a lot of stuff , references, and plotpoints will go over your head, or you will feel like you're left out. One of the major factions in NV was established in FO2.
Tactics is semi-canon, and is more of a top down tactics game like jagged alliance.