This question comes up a lot, in the end, it's really important that you play the breed that you most WANT to play. As for their differences:

Opifex: In the end, this is probably the best choice. They have the highest overall abilities and one of the cheaper ability/ip as well. You'll only lose a very few amount of nano skills by going opi, barely noticeable.

Solitus: This is probably the most popular choice. Better nano skills than an opifex, the character models aren't bugged like many of the opifex as well.

Nano-Mage: This isn't an option i'd recommend at all. They have much better nano skills but drastically less hp. The bad part is, at high levels, you'll have no problem casting everything as a solitus anyway, and solitus have a LOT more hp.

Atrox: Much lower nano skills, good amount higher hp. This would probably be one of the better choices if it weren't for the -% nano cost caps in Shadowlands.

In conclusion, do as your heart tells you. Opifex and Solitus are best suited to the profession, but Atrox and Nano-Mage can also do somewhat well.