I read that all SR 71 blackbirds were different, what was different with each one and is one regarded as superior to the rest? Why?
Like every military aircraft built over the years, the aircraft evolve. So to address the aircraft's history and all variants, let's refer to it as the Blackbird.
The Blackbird started out as the A-12 procured for the CIA. Fifteen of these were built with one being a 2-seat trainer. Two of the 15 were modified into M-21 aircraft prior to delivery. The M-21 was designed to launch the D-21 drones. The SR-71 at the Intrepid museum (60-6925) in New York was built as an A-12. One of the M-21's is at the Museum of Flight in Seattle with a D-21B drone.
Three high-speed interceptor prototypes were built and designated as the YF-12A and were capable of carrying 3 air-to-air missiles. Two crashed and the third is at the National Museum of the AF.
The interceptor was cancelled and the RS-71 program, renamed SR-71 after LBJ's misstatement at the press conference, went forward for the AF. Twenty-nine SR-71A's were built along with 2 SR-71B trainers. When one of the trainers was lost in 1968, the surviving aft part of one of the crashed YF-12A's was mated to the front part the SR-71 static test aircraft to create the sole SR-71C.
The question of which is superior is an opinion, but one can assume that the SR-71's were "better" than the A-12's as they came later building on what was learned from both the A-12 and YF-12A.
You may be referring to the fact that pilots felt that all Blackbirds seemed to have a distinct personality e.g. 962 was a hangar queen - a real lemon - whereas 972 never let you down. Blackbirds were virtually a hand built airplane but as to whether each one was different may have just been a matter of opinion.
There are a few variants of the SR-71 but there is only 1 that was ever used in an active military role.
The YF-12 was the SR-71's initial test airframe. Of the three airframes build tests were carried out by the Air Force and NASA. Notice the nose is slightly different than the eventual production version. (source)
A twin cockpit instructional SR-71 (SR-71B Serial Number 17956) airframe was built to train the pilots. (source)
As far as I know the SR-71 only had one airframe variant, the SR-71... (source)
The airframe was however capable of carrying a wide variety of payloads as dictated by the mission. You can find a nice overview of them here.
If you measure them in stealth, max altitude or speed. A-12 comes out on top. (not that stealth ended up mattering as Russians started to hook up computers to their radars and would then be able to spot the A-12s, they were never flown over Russia for that reason.)
If you measure them in surveillance equipment or range, SR-71 would come out on top.
If you measure in armament YF-12 would come out on top.