All three common processes (MIG, stick, flux-core) make strong welds on carbon steel when run correctly. Strength comes from the welder’s skill, the rod or wire selection, the prep, and matching the process to the conditions.
MIG is fast and clean in a controlled environment. Stick is rugged for outdoor and dirty conditions. Flux-core combines wire-feed speed with self-shielding for windy outdoor structural work. The right answer is the process that matches the job.