You could definitely say Mage Wars is the top of it's kind, as Cosmic Encounters is a totally different game centered around Negotiation (as I understand it, I haven't actually played). However, as far as "Games" overall are concerned, I don't think we can ignore his choice just because it's "nigh impossible" to supplant.
Saying it is the number one strategic game is fine though. The number one customizable game is hunky dory as well. Number one dueling game is a-okay. In short, Mage Wars rocks at everything except negotiation, lol.