Most of the time, imo, he's one of the few that's best using his own default weapon, simply because it requires no ammo and already has a pretty high dps.
But if you want him to keep his distance more, one of the high dps and/or unique rifles is a better choice. If you can keep him supplied with the ammo, of course. So one tactic could be default weapon for a lot of the game but collect/buy the ammo and give him something better for the mid-late game.
I wanted a non-gauss rifle for him because I have all the others using rifle type weapons (using a multiple companion mod) and thus Arcade is always the one that walks out to take point and always the first one to fall/be attacked...it becomes annoying. His relatively low hit points don't help.