Personally, I find it quite difficult to say. He's a very unique case.
Lesnar is always fun to watch in the ring. He had some great matches during his first stint in WWE, particularly against Angle, Guerrero, the Undertaker, as well as some fun matches with midcard guys such as the Hardyz. He's had a couple of famously dull matches, but he's usually able to put on a solid match with anybody.
His mic skills leave a lot to be desired, but he's had one of the greatest mic-workers of all time to fill that gap. His selling is excellent though, and he's one of, if not the, most realistic sellers in the company. Lesnar would definitely rank a lot higher for me if he didn't leave back in 2004, but alas.
I don't feel Lesnar belongs anywhere near the very top, though I feel he belongs around my top 30, give or take a few places. It's difficult for me as he's one of my favourites of all time, but objectively speaking, I wouldn't have him any higher than 25-30 at most.