What is the best filling for a cake ever?

whatever the client likes and is willing to pay for!

What are peoples opinions?

any flavor of SMBC !!!

I love whipped ganache, cookies and cream or the cookie filling. I guess it depends on what mood I'm in (= My kids love the cookie filling and cookies and cream.

I made some whipped white chocolate ganache and it is to die for as filling and icing. I put it on some leftover red velvet cupcakes, dipped them in some sprinkles and THEN had enough left to fill them and they are SOOOO good.

Chocolate bettercreme!!! (vanilla bettercreme with melted dark chocolate) Mmmmmmm, I am eating some right now and i'm in heaven!!!

Ok, seriously the question can't really be answered. Something is only "the best" in one's opinion, depending on your likes and dislikes. While I may love anything chocolate, I DO know people who don't like chocolate. So I could tell you "chocolate .... definitely chocolate" yet someone else could come back with "anything but chocolate .... definitely NO CHOCOLATE".

My husband thinks pumpkin pie is "the best" pie in the world. I dislike it so much that I dont' even allow it in my house.

Taste and preference.......

MONEY

my fav is strawberry buttercream

I recently folded "nutella" into some whipped buttercream and OH MY GOSH.... chocolate hazelnut heaven!
Tonight I am going to experiment folding "dulce de leche" into some buttercream! It is all about experimenting, having fun, and whatever "floats the clients boat!"
Happy Holidays!

My customers' favorite is Grand Marnier mousse. Kind of tastes like those chocolate orange candies.

My daughter eats the leftovers with a spoon.

As has been mentioned there is just not ONE BEST. It depends a lot on what cake it is being used with. Some combinations I love:
Chocolate cake w/raspberry
Butter Pecan cake w/bavarian creme
Lemon w/raspberry OR lemon fill

I'm not one for mixing fruit w/b'cream but I know of many who do.
I have never made curd but hear it is good.

Sooooooo there is NO one best!

I make a chocolate hazelnut mousse with marscapone and heavy cream. Sometimes I add some coffee to it and it is so yummy! It is my most requested filling.

My second would be my white chocolate raspberry filling. You can put that with a white chocolate cake or Lemon cake, or change it up with strawberries.

Toba Garretts (Sp) Chocolate Buttercream with ganache is to die for. I mean really, it's to die for. I don't always use the Godiva Liquor, but use coffee creamer instead, depending on the age of the client. I have also used Baileys in place of the Godiva.

I did a marble cake with oreo cookies and cream filling this week which was pretty good, I of course doctored the recipe as I always do.

where do i buy marscopone???

any grocery store should have it in with the feta and other specialty tub cheese

@SweetNSassyTreats: Go to the cheese section and hunt for mascarpone.

I love chocolate and I love raspberry, but I can't stand them together.

My favorite fillings are lemon curd and whipped caramel.