Alright, lots of wrong answers here :
a) Sauron cannot be killed without destroying the ring
b) Sauron wasn't "that" weak at the time, he was confident that his armies would succeed (which they would have if the ring had not been destroyed, with ease, the armies of both the Minas Tirith assault and the Barad Dur siege being "not much" to him)
c) Sauron's power was more than decent when the siege of Barad Dur occurred and it does not seem (after the books) that even Aragorn + Gandalf could have defeated him (again, power here is far beyond just raw power, it's about his control over the other rings, including the power of the Nazgul themselves, the shadow, the eye, the fear, etc.)
d) Sauron disliked the presence of Aragorn as he saw in him the risk of a second Elendil / great alliance. This is much more related to his ability to bring light and protect Sauron's enemies from the paralyzing fear than to his relative combat potential (all of Aragorn's major wins against Sauron are leadership wins, convincing the Rohirrim, convincing the ghost army, etc.)
e) Sauron did not care about that fight, the foolish alliance army was going to be crushed in front of his gates so why even bother.
- Also I don't know where or what, but someone said Gandalf and Sauron were both Maiar and should be more or less on the same level ..
Well that's just wrong . Sauron was probably the greatest of the Maiar, just as his master Melkor (Morgoth Bauglir) was the greatest of the Valar. For much weaker Maiar, just take a look at Radagast (lotr) or the Silmarillion/Unfinished Tales (much about weaker Valar in there too).
Among the Istari, it would seem that Saruman was once the most powerful, got replaced by Gandalf during his fall - and that there were other weaker Istari.