To me, this is a blunt and obvious answer. I was maybe 16 when I got my first "wheeler" as my brother would say. I worked hard and saved my money and finally got the courage to ask my neighbor if I could purchase his ATV from him. With no knowledge on what it was, what was wrong with it, or how I would get it home, I actually hoped the neighbor would tell me no. I was sad but partially relived when he said no and I walked home. The next day my neighbor sought me out and told me I could take the ATV for free, if I could get it home. Without even thinking I started to push it down the road flat tires and all. 
It took me about 3 months and 300$ to get it running which was a small cost for the many memories shared with it. Me and my brother made our own paths, mud holes, and ramps just because we could. The ATV never got stuck. It had 2wd, 4wd and diff lock (tank mode). The oil was never changed because simply it didn't need it. It ran on muddy water and lots of heart. To this day I have no clue mechanically how it roamed this earth defying every mechanical rule human'S know. Anyways, the answer, the best ATV ever made is a 1996 Suzuki quad runner 250cc... If any of you have different thoughts, let me know in the comments!