Wasn't even the best AKI game of that generation. Virtual Pro Wrestling 2 beats it easily. That game had MMA, a ridiculously stacked roster, and a deeper creation mode. No Mercy was freaking great though, I have a ton of love for it.
Fire Pro Wrestling Returns deserves a mention. The graphics can put a lot of people off, but the depth of that game was mindblowing.
VPW2 and No Mercy both had engine elements the other didn't have. Overall VPW2 comes out ahead in that regard but overall I think it's very hard to make a slam-dunk case for either being "easily" better than the other. Did VPW2 even have backstage areas ?
Either way, my (personal) Mt. Rushmore of rasslin games so far is: No Mercy, Here Comes the Pain, WWE 2K16
I know it's probably insanely early to put 2K16 in there, and maybe I'm suffering from recency bias ... but I think it objectively has the most realistic match pacing of any wrestling sim I've ever played. Hell, it's the first wrestling game I can say really is a sim. Most of the Smackdown-lineage games are arcade/simulation hybrids I'd say (some leaning more to one side or the other), but ... for example, even a squash match on this with a very high rated character against a very low rated character lasts a few minutes. In No Mercy I can get Austin up to special in a few seconds, and three Stunners in a row is enough to pin anyone. Now I guess someone could argue that for some characters that is realistic, but ... anyway.
With a better Career mode and just a couple/few other extra bells/whistles, 2K16 ... well, I doubt anything could really surpass No Mercy for me just because of the life-events going on at the time that I associate with No Mercy, but I might be considering the possibility at least. But I've been putting more time into this than any other game I've bought in a real long time, and I don't think that's just random coincidence.
I considered putting Day of Reckoning on there ... it was a long, looong time before a game with the AKI-engine lineage felt like it really captured the spirit of the N64-era games, and DOR 1 at least felt like a worthy successor. But, idk ... maybe it's because I never played that as anything but single player (although that doesn't appear to be harming my love for 2K16).
I feel kinda bad for some of the other Smackdown games. I do think HCTP is the one I put the most time into, but a lot of that is, again, due to life circumstances ... was hanging around a lot of other rasslin fans during the HCTP era, heavily dried up on that afterward. And had started to get a lot more into mma and training with my buddies, and some other nerd stuff, kinda pushed rasslin to the backburner for a while.