I have to agree on Wildcat being the toughest mud race I've ever done. I've done a lot of mud races, but that track was nearly impassable it was so bad at some points.
Baja is gnarly. I love the lower section that is less sandy, hate the upper deep sand parts. The thing I really hate about Baja is that the slower you go, the worse it is. I'm always exhausted from there just because I can't coast like I'm used to!! haha The thing is though, for us Ohioans that grew up riding hard pack Baja is brutal. For those that grew up there riding sand they think it's easy, just pin it.
Glen Helen was tough -- I remember being there and looking at it in person and being unsure if I was actually going to make it up some of the hills. It was very intimidating. Pala national track was huge and intimidating as well, mostly due to the sheer size of the jumps.