I need a good recipe for a buttercream that's not too greasy/tastes good, but also holds up well under fondant covered cake layers. Thanx in advance!

My basic buttercrem I use for everything- filling, icing, decorating, crumb coating & under fondant.
I'm not one to measure my BC (I know crazy for a baker!) I know it by taste & texture, but here are the instructions for one 32 oz bag of powdered sugar.
Cream together 1 stick unsalted butter & 1 stick margarine with a dash of vanilla and a dash of lemon juice (probably close to 2 tsp vanilla & 1 tsp lemon juice. add the powdered sugar 1 cup at a time on low mixing the whole time, adfter all the sugar is combined add a litle milk (probably 1-1 tbsp) until you get the right consistency, then whip on med/ high (6 on my kitchenaid) for 1 min. Add more milk to thin out more and get a smoother icing, add less for a stiffer icing for decorating. You can also substiture almost amy extract for the vanilla- almond is really good too.

I'm relatively new to cake decorating, but I think it depends on where you're located. I really like a recipe with butter, but I've found when it's warm and humid, it melts. If I use a shortening only recipe (I prefer hi-ratio) it holds up better. I think they're both good, but I think shortening only is a little sweeter. I use the Wilton recipes and I haven't had any issues yet.

You should try sugarshack's recipe and indydeb's recipe, they are both great tasting and do well under fondant for me.

My white velvet buttercream recipe gets rave reviews. It's in the recipe section here on CC. It's listed as Jibbies White Velvet Buttercream.

Jibbies

I use the buttercream dream recipe which is butter & shortening. If it is a super hot Louisiana day and I'm traveling with a cake, then I use a straight shortening recipe.