I don't want the cake to be too sweet, but I'm not sure what would taste the best.

If you are not covering the cake in fondant I like a stabilized whipped cream icing.

If you are covering it in fondant I would use a lemon buttercream.

If it isn't going to be a decorated cake, you could use a lemon glaze.

The other suggestions sound really good, maybe champagne buttercream to add another.

I think an original cream cheese icing would taste great!!!

I use IMBC on mine and flavor it with a little lemon curd.

White Chocolate BC is what I like.

I second the IMBC, the flavors compliment eachother so well!

Yes, the imbc or smbc with the lemon curd would taste great!!

RLB has a recipe called, "Woody's", that is my white chocolate base cake. It is on the web and in her Heavenly Cakes book. As written, it is a two layer lemon cake. I torte to four, add a filling of gourmet raspberry jam, and cover it with IMBC/curd. It's my mom's favorite and her birthday cake of choice.

Edit: I forgot to mention that this cake, filling, and frosting is not overly sweet at all. It has a well-balanced flavor.