If you're a fan of HK specifically or want a dps companion I think Agro can make the best craftable droid parts.
If you are looking for an uber-companion I'd suggest gearing up Treek though. She can wear player-equipable armor instead of droid parts. That being the case I put my commando healer gear and vanguard tank gear into bind to legacy stuff. She is also unlockable for your whole legacy for a single fee. End result is a sweet tank and/or healer companion at 55 for all characters in your legacy without having to reproduce the same droid parts for every iteration. In tank mode, which is usually what I run her in, my Treeky is a 40k tank with off heals... so rad.