For me it is like this:
1. Lendl
Lendl revolutionized power tennis - when he hit his peak in 1984, it was the era of graphite racquets where the surfaces mattered more than before. Lendl was a bit unlucky here - Australian Open switched to hard courts only in 1988. This should have been his favorite surface as AO's hard courts were then considered to be slower than USO's courts (Lendl won three USOs). Despite being past his peak, he still won AO 1989 and 1990 while finishing runners up in 1991. He also underachieved in his pre-prime years and could not win a slam until 1984. Still, his record of 8 slams is very impressive. He had crazy competition throughout his career from the likes of Connors, McEnroe, Becker, Edberg and Wilander among others.
2. Agassi
A lot of people underrate Agassi because he won 'only eight' slams. But Agassi won all or most of those slams when the surfaces were far apart. During the period from the mid 80s to the early 2000s, it was virtually impossible to win both Wimbledon and FO but Agassi did it. In fact he was the only player to win Wimbledon and FO between 1981 and 2007 (Wilander won AO on grass but not Wimbledon). He even achieved the career grand slam. At no point did that achievement matter more. He foolishly skipped AO in his early years but went on to win 50% of his titles there. Like Lendl, that was his favorite surface. Remember Agassi is the only player to win the career super slam (grand slams + YEC + Olympics and Davis cup as well). Agassi faced insane competition at the start of his career with the likes of Edberg, Becker, Sampras and Courier. Later, his competition was only from Sampras.
3. Connors
Jimmy Connors peaked at a time when the competition was weak. He had a virtual free run in 1974 and 1975. In fact, his opponent in two slams in 1974 was a 39 year old Rosewall. The legends of the past like Laver, Rosewall and even Newcombe and Ashe had gone past their primes in the mid 1970s (but Connors lost both to Ashe and Newcombe in 1975!). Bjorn Borg was pre-prime until about 1977. Once Borg hit his prime, he made Connors his bunny and Jimmy could win just one slam from 1978 to 1981. By this time, McEnroe also arrived. It was only when Borg stopped playing in slams in 1982 that Connors started winning slams again. To his credit, he made an amazing comeback in 1982 and 1983, winning three slams. He had good consistency and longevity but his slam winning run had stopped in 1983. He was unfairly prevented from playing at the FO in the mid 70s, otherwise he could have won the grand slam in 1974. But considering how he fared against top competition, I will put him at number three.