If I were you I'd keep mobility in mind. All these mechs can do mid range combat very well, but have different playstyles. If you don't want to lose too much on speed compared to the Heavy and Medium class, go with the Victor. If you don't care about the lack of speed the other 3 offer more pure power.
The Atlas is excellent though you have to get used to being a magnet for enemy fire. It offers many builds for any possible playstyle. Also DDC brawlers kings of the hill when it comes to pure firepower.
The Stalker is more suited to long and mid-range direct fire rather than in-your-face brawling because it's lacking in maneouverability.
The Highlander is second only to the Atlas in close-up face smashing but is unmatched in it's ability to lay down it's awesome firepower and get away with it tanks to JJs. It's also surprisingly mobile.
All in all, if you can't decide I'd go with the Highlander it can do so many things and JJs are OP.
