How to choose within this category
Start from the question almost nobody asks first: how much engineering management do you already have in-house? Every option above splits cleanly on that axis. Marketplaces (Toptal, Strider, Terminal), direct platforms (Upwork, Contra), and dedicated-team firms (BairesDev, Andersen) all assume you can write a sprint plan, review pull requests, and tell good work from plausible-looking work. They add capacity to a function you already run. A solo full-stack builder or a boutique agency instead owns an outcome — you hand off a scope, not a backlog.
So the real question is not "who is best." It is "who fills the gap I actually have." Missing hands but you have a technical lead: a vetted contractor network. Missing the whole engineering function on a 0→1 build: a single accountable owner. Missing a team for a funded multi-quarter roadmap: a dedicated-team partner. Match the shape of the help to the hole, and budget stops being the first filter.