Personally, I think New Vegas is the best game in the series, followed fairly closely by Fallout 2 and 1 in that order (I haven't played 4 or 76, but from what I've heard they'd be unlikely to break into my top three). Fallout 3 is... alright, I guess, though the fact that I've only played through it once compared to the many, many times I've played 1, 2 and New Vegas might speak for itself.

I do think Fallout Tactics, while flawed, is somewhat underrated, though it would probably be a weird way to get into the series, considering how different it is from the others.