1. Preheat the oven to 350 F.
2. Prepare the crust: Combine the graham cracker crumbs and butter in a mixing bowl and mix to form a crumbly dough. Press the mixture into an 8-inch pie pan. Bake the crust for 5 minutes. Remove the crust from the oven, but leave the oven on.
3. Meanwhile, prepare the filling. Combine the egg yolks and sweetened condensed milk in a mixing bowl and beat with a mixer at high speed until light and fluffy, about 5 minutes. Gradually beat in the lime juice and zest.
4. Pour the mixture into the crust. Bake the pie for 6 to 8 minutes, or until the filling is set and an inserted skewer comes out clean and hot to the touch. Remove the pie from the oven. If choosing the meringue topping, increase the temperature to 400 F and proceed with Step 5. If choosing the whipped cream topping, turn off the oven, set the pie on a rack to cool room temperature, and skip to Step 6.
5. Prepare the meringue: Beat the egg whites to soft peaks with a mixer, starting on low speed, gradually increasing the speed to high, and adding the cream of tartar after 20 seconds. Beat in the sugar in a thin stream and continue beating until the whites are glossy and firm, but not dry. Spread or pipe the meringue on top of the pie. Bake the pie until the meringue is nicely browned, 3 to 5 minutes. Let the pie cool to room temperature. Refrigerate, uncovered, for at least 4 hours before serving.
6. Prepare the whipped cream topping: Place the cream in a chilled bowl and beat until soft peaks form. Add the confectioners' sugar, vanilla, and lime zest and beat the cream until stiff. Spread or pipe the whipped cream on top of the pie. Refrigerate, uncovered, until serving. For the best results, serve within 1 hour of adding the whipped cream.