Even though Edberg had a horrible overall record against Becker (10-25, though 3-1 in Slams, but then again, like 0-3 in not very close Davis Cup matches), I think they compare favorably overall in terms of legacy and achievements. I would put Wilander right on par with both of them. All of them won 3 of the 4 slams (on at least 2 surfaces) and did it over a substantial amount of time. (Wilander first slam '82, last '88) (Edberg, first '85, last '92), Becker (first '85, last '86). All won a lot of other tournaments and Davis Cups.
All had identifiable personalities/plating styles. Wilander was a reserved backboard. Edberg, a great sportsman and classic serve and volleyer. Becker was a power player, passionate, a bit volatile.
Wilander shut it down much earlier, but he's also the only one to win Slams on three surfaces (correct me if I'm wrong, but he won the AO on grass, right?)
Anyway, that's one hell of a second tier.
Agassi is right on the cusp of first and second tier. With the career slam and 8 overall, he's clearly ahead of this group, but would be at the bottom of the Tier 1 group.
Under this Tier 2 group, you get to players like Courier, Kuerten, and even Chang, who although he only has one Slam, made huge impact and won a ton of other tourneys and reached other Slam finals.
Just my opinion.