Boba didn't have bad aim. He wasn't trying to hit Luke, he was trying to cover himself.
Think about it. He knew that Vader wanted to catch Luke and was part of the process to lure Luke to the carbon freezing room. Had he killed Luke there...well, wouldn't Vader be a little pissed? But had he kept walking like normal, he might have been shot by Luke. So he shoots in the general direction of Luke as cover fire to avoid being shot and keeping Vader's prize intact.
I'm pretty sure Boba underestimated Luke on Tatooine (who at that time was looking like a pretty poor Jedi) when his gun was cut in half and shots were blocked when he had Luke tied up. He was about to kill Luke though with a missile before Han hit him with a fluke swing. Not his greatest moment, but he recovered from it. Let's not forget that Luke turns out to be one of the greatest Jedi ever.
As for IG-88, we have to reference all four to be fair.
IG-88A: Destroyed by Lando and Wedge when Death Star II blew up.
IG-88B: Destroyed by Boba Fett on Bespin. Boba 1, IG 0.
IG-88C: Destroyed by Boba Fett in space. Boba 2, IG 0.
IG-88D: Destroyed by Dash Rendar of the Rebel Alliance.
While IG-88 was awesome (most people don't know that if the Death Star hadn't been blown up, IG-88's robot revolution would have been much worse then the Empire was for non-droids) he fought Boba twice and lost twice. Therefore, not as good as Boba.
Jango was indeed awesome, but Boba had all of Jango's weapons and skill sets, and made improvements on everything. He became better then Jango which is noted in the EU. I understand if people like a certain bounty hunter over another, but the characters in the SW universe acknowledge that Boba is the deadliest and he proves it time and time again. The movies downplay his greatness because most of Boba's lore is established in the EU.