John Wick Quadrilogy:
John Wick - Keanu Reeves stars as the titular John Wick, a retired hitman mourning the loss of his wife, who then goes a revenge killing spree after Russian gangsters kill the dog his late wife left him as a companion, and, steal his classic Mustang. Very entertaining throughout, well paced with some very good action.
John Wick: Chapter 2 - picks up straight after the first film, and begins to establish more of the lore of the "High Table" - an unseen cabal of the world's leading gangsters to which John previously served. Introduces Larry Fishburne's "King of the Bowery" character, for a nice little Matrix "reunion". Again, the action is very good, pacing is good, and it is stylishly executed.
Parabellum: John Wick 3 - Ok, so this is where it all starts to go off the rails a bit for me. Apparently the streets we walk are littered with hitmen, and nobody seems to really care all that much when virtually open warfare erupts on major city streets. The "High Table" seems to have an infinite number of highly disposable goons for John to dispatch, and John seems to have an energy and stamina that would put any PCP/Speed addict to shame. Mark Dacascos is probably the best thing in this film, which while entertaining, verges into the silly and loses any notion that this series is even remotely grounded in our reality, that thought is thrown well and truly out of the window.
John Wick: Chapter 4 - Where to start? So, this is basically three hours (yes, three hours) of John Wick executing an unending stream of High Table goons in a last attempt for John to free himself of said High Table. The executions come so thick and fast, that by the end of the first hour you become numb to it and can't help but wonder how the hell Wick has expired from a heart attack. Donnie Yen is excellent throughout as a blind assassin sent to kill John, and is a real bright spot. There is also a very good action sequence set at the Arc De Triomphe. Outside of that, this is too long and borderline boring. The 13 year-old me may have found this endlessly amusing, but the older me was not impressed.
Overall, the first film is by far and away the best and the fourth, easily, the weakest. If you've not seen any of these, then I'd recommend the first, maybe the second, then after that you're really not missing much.