Keith Courage was probably the first for me, judge me if you must but that whole rainbow mech transformation was awesome to me at the time! First time the Turbo snagged me and showed me the style of games I could expect. I got games on release as they trickled out, and I remember Bonk's Adventure, Bonk's Revenge, and Dragon's Curse in particular being pretty magical experiences.
I think the real deal is when the Duo came out, and I was completely into all the pack-in games. I had been drooling over the promotional magazine they released for months and I was particularly taken with the screenshots of Ys I&II. I had a job with my then brother in law mowing lawns, so I was able to save up my own money and actually earn it myself. Between the satisfaction in that and the suspense leading up to actually getting the thing, I was really stoked. The pay off was awesome too, because between Ys I&II, Ninja Spirit, and Gate of Thunder alone, you really got some awesome games. Bonk's Adventure and Revenge I already had on Hucard, but cool editions nonetheless. I didn't have Bomberman though, so that was also cool. So much value packed into that box! I would play GoT with my Brother-in-law when he came by for hi-scores, and at night I'd work on Ys. I was totally blown away by the full screen character portraits and voice acting, CD audio in games was new to me at the time. I will always regard the original Duo release as one of the best pack-in values in the history of games, not only did you get a lot of them, they were also all outstanding games.
