This is of course a highly subjective question, and I would encourage you to read the play yourself and/or watch a production of it so that you yourself can pick what the best part of this play is. The Al Pacino version that came out in the last few years is an excellent film adaption, and gives a real flavour of the play. Do watch it, as you won't regret it.
However, I think the best part as far as I am concerned, and I am sure the best part for a number of others as well, would have to be the court scene in Act IV scene 1. This is of course the climax that the whole play has been leading up to, as we see a grim Shylock determined to get his revenge on Antonio, who has so wronged him in the past. The way that the tension mounts right up until the point when Shylock is going to cut the pound of flesh from Antonio until Portia reveals the loophole that condemns Shylock is masterful:
Tarry a little; there is something else.
This bond doth give thee here no jot of blood;
The words expressly are "a pound of flesh."
Then take thy bond, take thou thy pound of flesh;
But in the cutting it if thou dost shed
One drop of Christian blood, thy lands and goods
Are by the laws of Venice confiscate
Unto the state of Venice.
Suddenly we see a huge reversal in the situation. Up until this stage everything appeared to be going well for Shylock. Now, suddenly, things start turning against him and become worse and worse. This scene interests and excites the audience as we watch the seemingly inevitable fate descend upon Antonio, and then only at the last possible moment descend upon Shylock. Pure Shakespearian genius.
I believe that the best part of the play is the trial scene. It is the climax of the play, with all of the previous conflicts meeting here.
i dont kown