It's almost easier to say which are the weakest. The Mourner, The Handle, Butcher's Moon and Ask the Parrot are easily the least successful.

The Hunter and The Score are the pinnacle. You can probably add The Rare Coin Score because it introduces Claire.

I might add The Handle and Slayground because they both have Grofield in them.  And Slayground is pretty off-formula.  

Pretty much What Slam says, though I'd add the Man with the Getaway Face, the Jugger and the Outfit, and the best part is that unlike a lot of other book series out there you don't really need to read them in any order.

I still have a couple of originals to go but I agree with the consensus: quick, readable, simple fun. My recommendation would be The Black Ice Score. Westlake stumbled into something much more substantial, perhaps accidentally as he wrapped up the tale in typical Parker fashion. Along the way, though, Parker came across some interesting characters in a particularly interesting predicament. It holds up.

They Shall Have Stars
by James Blish
c. 1957.. originally called 'in the year 2018'  My copy is part of a compendium called 'Cities in Flight' c. 1970, but I thought I'd review each separately.

As I've come to expect from James Blish, some real thought provoking stuff both Space Travel, and the possibility of immortality/super longevity.   What really struck me as unusually prophetic was that he describes that no one has cared about Space for 15 years'.. set in 2018.  Granted, they got alot farther in his world (out to a pretend 10th planet), but the similarity to real life was striking.

Of Course, since it was written in 1957, we have Communists as the main bad guys, and a Joe McCarthy-like government man that is the enemy of progress.

The story is pretty fragmented, with one of the characters a spacer on leave that stumbles into a company researching an anti-aging vaccine, and the other taking place on 'the Bridge' in Jupiter, which, apparently is there to really test anti-gravity theories... though no one knows it.   It's a good read for it thought provoking-ness, but the plot is more of a set up than a fully realized story, which, based on a bit of research, doesn't get realized.

Upon reflection, except for the end of the last sentence that is the same synopsis that appears on the Parker page of Kevin Burton Smith’s Thrilling Detective website. It’s a site worth checking out if you have any interest in the genre. Its short fiction varies in quality because they are contributed, not purchased, but otherwise it is an excellent reference:

Parker

Thrilling Detective

I'll have to check that site out.. sounds neat!

A Life for the Stars
James Blish c. 1962

This is the latest written of the 'Cities in Flight' stories, though 2nd in the compendium (I assume chronologically).   This one is mostly a coming of age story for Chris DeFord, a young hillbilly living off the land (as best he can) with his family in Appalachia.   He gets shangaied by Scranton, PA when it 'goes Okie'... in Blish's 32nd century, most of the cities of Earth fly around the galaxy to provide manpower and industrial expertise for the various rustic human colonies.   They pass him off to New York to give themselves less mouths to feed, and there he learns about Okie culture and society.  

Eventually, New York comes into contact with a down and almost out Scranton, and Chris saves the day.

The strength here is definitely Blish's unique future, one in which the Cold War never really ended exactly, but rather the west became so much like the East to try to fight them, the Soviets won by default.. leading to 1000 years of stagnation and exodus from Earth.   THe heroes of the 1st book are mentioned as saving the humans and getting them out into space, until their 'Spindizzy' (Blish's nearly omnipotent interstellar drive) was re-discovered 350 years later.  They mention aliens, but only in passing, as they're apparently quickly defeated by the flying cities.  Sadly, there's not 'Federation' or anything here, only the remnants of the people that conquered the aliens (who call themselves the Emperors of Space), and Earth, who's main role (after the Communists finally fell) is to police the cities as they fly about.   It's a clear cautionary on making sure you don't kill freedom while trying to save it, and as dour a future as I've read in a while...even the ones that have some version of World War 3 generally get redeemed eventually.   It seems a fertile ground for story telling though, so I'm excited to read the other two books (well, one book  and 3 short stories tied together as a book).

Earthman, Come Home
by James Blish

So this was the first on the 4 books written, and, to be honest, if I'd read it first I probably wouldn't have read the others. Where the other two installments are high philoposphy with some engaging sci-fi mixed in, this is very vanilla. It chronicles the adventures of New York and it's mayor, Adelfi. The problem is, he's thoroughly unlikeable and manipulative. His 2nd in command (the kid from the other book is killed in between the two for making a bad decision) is just as much of a bastard.

There are some fun bits, like when they use a planet like a cue ball to bump a warship away from across the galaxy, it's just not that great of a story.

Worse, still, many of the details that were lovingly laid out of the history of this world of travelling cities are different, and make far less sense. Overall, very disappointing.

And finally....

Triumph of Time
by James Blish

After New York's travails, then land on a distant planet and take over, only to have the 1st planet they launched (in the previous book) turn up with a suddenly (well, not too suddenly, it's about 150 years later) high tech culture that has discovered the universe is going to collide with the anti-matter universe and reset itself in a couple years.

They race to the center of the galaxy to try to control the process, and seem to succeed, but its not entirely clear.

While the addition of a couple new children of the orginal characters added a little something, it was still pretty lackluster compared to the early offerings, and the lack of ending was just silly.