What are some good Makeup Essential Must-haves?

I'm planning on buying some new makeup from MAC. and Urban Decay but don't know what to get from what brand. What are some makeup essentials that you will only buy from MAC. and some makeup essentials that you will only buy from Urban Decay? Also I'm wondering if it's worth it to get Urban Decay's Eyelash Primer Potion and Urban Decay's Lip Conditioner Primer Potion or should I just buy the Eyeshadow Primer Potion? I'm not much of a lip gloss girl (as most lip glosses I've tried are too sticky). Also should I buy my eye makeup brushes from MAC. or Urban Decay? Lastly what are must have eye makeup brushes from either brand? Thank you!