The original is still the best by some distance. Die Hard with a Vengence is very good, aside from the slightly limp ending, and Die Harder is better than people think, I think the problem is that its just a bit too knowing and self referential.
The fourth one I think is actually ok, at least until the last half hour or so, but the fifth is just terrible, not even so bad it's good, it's just dull, Jai whatver his name is is terrible as mini McClane (in contrast to Winstead who makes for an awesome Lucy) the worst thing about it is that it doesn't even feel like a Die Hard film, it's just a bland action film where both the heroes happen to be called John McClane.
I can appreciate they wanted to scale things back a bit (terrorists take over an office building, terrorists take over an airport, terrorists take over a city, terrorists take over America, what next the world?) but you don't have to lose the essence of what makes the series great, Mcclane's always been a fish out of water, so sticking him in Moscow should have been great, he doesn't know the language, he doesn't know where he is, but aside from 30 seconds in the back of a cab he might as well have been in Chicago.
I also think it's worth noting that the better films have had a much wider cast, great characters like Al, Holly, Dwayne T Robinson,the airport cop, Dick Thornburg, Agent Johnson, Special Agent Johnson, Ellis...hell his fellow cops in Vengence feel so real you can almost imagine a TV show that just follows them.
I'd like to hope they could pull a Rocky Balboa with the sixth one (and if it isn't called 'Old Habits Die Hard' someone wants shooting) but I fear it'll be shit.