i am currently on week 5 of athlean xero program, for me i found it to be the most challenging workout regime i've ever done. combined with the included xtinction-abs and meal plan, you can't go wrong. i did weeks 1&2 then repeated them, then same for weeks 3&4, then weeks 5&6,then will repeat all over again. i have also been including loaded pack leg workout 3 times a week to strengthen and condition for elk terrain. i tried the p90x and it just didn't work for me.
I'd add a caveat or two. Gaining muscle is all about calories and protein, the rest of the diet can be less than that I'd consider optimal for long-term health. Rippetoe's GOMAD (gal of milk/day) pretty much proves my point. Rip says eat Big Macs if you want, just drink the milk and keep lifting more weight each week. Gaining a lot of muscle and being lean does require an optimal diet, and most agree you can't work on both simultaneously, though long-term crossfitters do tend to gain a little muscle and lean out.