My advice would be get to a shop that sells a range of brands. I assume that you are aware of Rab, Montane, TNF, Patagonia, and Paramo, to name but a few.
Take your layers with you, if you have them yet, and ask to try on some jackets, or if ordering online get the next size up from your usual.
If it is Everest base camp that you are talking about you don't want the garment to be too light weight as the material may possibly not stand up to the rigours of the trip.
I use a ME Rupal, it's very tough and I can get a good synthetic jacket under it, or even over it if necessary.