Herb Trimpe is really sweet and humble. Over 2 cons, he signed a bunch of stuff for me (an issue of Backissue Magazine that had an article about him, Special Missions #1, some of my favourite Transformers covers that he drew, his first issue as the Transformers' artist, even al 4 issues of Order of Battle), all for free. As for what's best, bring him something which is important to you that he did, like your favourite Special Missions issue.