Unicron or Primus

[GoktimusPrime]

Primus is a better toy as he isn't a big shellformer like Unicron is, and his planet mode is a lot better. But to be fair, HasTak had more creative freedom with Primus; only his planet mode is based on a pre-existing design from G1 (planet Cybertron) - since Cybertron never transformed into Primus in G1 or any other pre-Cybertron canon, HasTak just created a new robot mode for Cybertron Primus. However Unicron of course has had a long history in Transformers; both his planet and robot mode are iconic and widely known amongst fans, and HasTak had to try to engineer a toy that would transform between these two modes and try to maintain visual fidelity. And on top of that build in Mini-Con activated gimmicks (since he is an Armada toy after all). Considering all that I think Unicron is well designed and in that context on par with Primus.

If you just want a big Transformer that's well engineered then go for Primus. From a G1 (or Armada) fan's POV you might consider Unicron. If you want a really BIG Transformer, then go for any Transformer whose name ends with "Maximus." ('cept Juniors :P)