Reds and pinks - Red, ruby, baby pink and rose pink.
There are probably other colours which I would call favourites, but for the purpose of answering this question these are my suggestions. I would be surprised if you could stop at 20. I warn you they are addictive!
To go with the promarkers I use digital, ultra smooth premium cardstock from Craftwork Cards.
The Passion for Promarkers blog is a great place to visit for challenges and prizes related to these fabulous pens. and the Promarker Weekend Workshops blog is also a very useful blog, for information about shading and other effects achievable with these pens.
Cardinal Colours has kindly provided ten promarkes for the winner of November's challenge.