There's little possibility of selecting one of the three films in this trilogy above any of the other. Even though each of the three is slightly different in tone, they really have to be taken as a whole. Funnily enough, the last of the trio 'The Good, The Bad and the Ugly' (GBU) should really be seen as a prequel to the two films which were released slightly earlier. Clint Eastwood's character: Joe, Manco or Blondie (take your pick) is the obvious connective tissue between the three films, but GBU is obviously set earlier as it shows Eastwood's character slowly gathering the props that make up the character we see in the two earlier films. Also, in 'For a Few Dollars More' (AFDM), Lee Van Cleef's character is obviously a 'veteran' of the Civil War in which GBU is set.
Sergio Leone's Dollar's trilogy, was pretty much virtually responsible for the brief but successfuly 'cottage-industry' that blossomed in the wake of the release of A Fistful of Dollars (AFFD). There was a flurry of very similar films (in terms of story, or cinematography and/or music) released to capitalise on the success of AFFD, but very few of the hundreds of 'Spaghetti Westerns' as they came to be known, went on to have anything like the renown of the Dollars Trilogy.
It's pretty strange to realise how poorly these films were received when they were initially released. Even now, it's pretty obvious that the production value was pretty low on A Fistful of Dollars (the constantly out of sync dubbing of the soundtrack being one of the most obvious elements), but with the latter two films and particularly The Good the Bad and the Ugly the 'low-budget' (even for the times) look and feel of them seems to be less obvious. By the time I had grown old enough to become aware of these films (my older brother was an afficionado of the 'spaghetti western', so I came to know these films via his love of them) they had already gained their status as ground-breaking pieces of cinematic art in their own right.
Leone's three films though, stand apart from the majority of films that followed in their wake, mainly because of Eastwood's now iconic portrayal of 'the man with no name', which made them so effortlessly cool. Despite the casual violence and moral ambiguity of the heroes of these films, at the end of the day they were honourable men who operated in environments that militated against any kind of honour at all.
The second-wave of 'spaghetti-westerns' that started with Enzo Barboni's also excellent 'They Call Me Trinity' was more short-lived than the first and to some extent less influential as they moved away from the more hard-boiled storylines into more comedy-themed stories.
How much of the Dollar's Trilogies' success rests wholly with Leone is a little hard to say almost fifty years after the event, but without doubt the film's somehow managed to bring together a surprising amount of talent for very little money and arrived at just the right time to capitalise on the decline of the Western genre in the US and elsewhere. Eastwood was not Leone's first choice, the part being first offered to (a littany of other actors including Steve Reeves, Charles Bronson and Henry Fonda) who all turned it down for one reason or another. Eastwood though, without doubt brought the character to life and it would be very hard to imagine anybody else playing it now. Leone also had at his disposal one of cinema's greatest composers in Ennio Morricone (credited as Dan Savio in AFFD) who created the now famous guitar riff and whistle that punctuates all three movies at various points.
The three films are constantly referenced as one of the best movie trilogies ever and GBU, is also quoted as being the best westerns of all time by some critics and film-makers alike. The dollars trilogy is still being referenced by modern film makers to this day (most famously in Back to the Future - Part III (1990) but references can also be found in Lock, Stock and Two Smoking Barrels (1998), Lord of the Rings (2001), Hot Fuzz (2007) to name but a few). That in itself is some achievement.