I may have asked the question in an odd way, but I am having trouble understanding the most defining differences when it comes to the Good.
Plato in the 'Republic' (VII, 532a-b, 534b-c) sees the best of all things - the Good - as existing separately in the Form of the Good (auto to agathon). The Form of the Good is a separate entity, transcendent of space and time; and everything else that is good to any degree is so by virtue of its 'imitation' of the Good.
Aristotle (Nicomachean Ethics 1.6; Eudemian Ethics 1.8) criticises the Platonic Form of the Good. There are many things that are good, he explains : a quantity can be good, a quality can be good, a place can be good, a position can be good, a relation can be good, a time can be good. But there is no credibility in holding that the goodness of a time has anything in common with the goodness of a place or of a quantity. There cannot be a common property of goodness which all these diverse things share. What makes a place good is one thing, what makes a time good is another.
Aristotle differs from Plato in this way and, in what logically follows from this, in denying that the Platonic good exists at all. There is and could be no Form of the Good.
But he does believe that 'the good, that is, the best' (to agathon kai to ariston' - 'Metaphysics', 12.10) is real. It does exist, only it is nothing like Plato's Form of the Good (even if that did exist). At 'Nicomachean Ethics', I.1, he says :
If, then, there is some end of the things we do, which we desire for its own sake (everything else being desired for the sake of this), and if we do not choose everything for the sake of something else (for at that rate the process would go on to infinity, so that our desire would be empty and vain), clearly this must be the good and the chief good. [tr. Ross]
What we desire purely for its own sake is 'eudaimonia', happpiness or (better) well-being or human flourishing.
Aristotle's view is that human beings have certain characteristic activities and that eudaimonia - our good - consists in exercising those activities as perfectly as possible. In doing so we fulfil our 'ergon' or function.
Our ergon or function is a practical life informed by reason. This leads to the doctrine of the mean (we observe the mean when e.g. we act courageously and not on the one hand in a cowardly way or on the other foolhardily). Phronesis or practical wisdom yields knowledge of the (rational) mean in our actions. Reason is capable of various forms of theoretical and not merely practical development, however : scientific knowledge, intuitive understanding of first principles not least.
One caveat is that Aristotle seems to be divided between a practical 'take' on the good - that it involves a practical life informed by reason - and a different, contemplative view developed or sketched in 'Nicomachean Ethics', X. Here contemplation (theoria) is projected as the ideal life. This is a crux in Aristotelian interpretation and can't properly be examined here.