Was Crocodile stronger at Marineford? Or was he holding back in Alabasta?

 During the Alabasta arc, Crocodile displayed a level of power that was initially considered overwhelming by the Straw Hat Pirates. He possessed the Logia-type Devil Fruit called the Suna Suna no Mi (Sand-Sand Fruit), which granted him the ability to control and transform into sand. He had a reputation as a Shichibukai and controlled the desert kingdom of Alabasta from the shadows. His strength was showcased through his battles with Luffy and others. At Marineford, Crocodile was present as part of the war that took place at Marine Headquarters. While he did participate in the battle, he didn't display the same level of dominance as some other powerful characters present. This has led fans to speculate that he might not have been as strong as initially portrayed in Alabasta. It's important to note that power scaling and character abilities can be subject to interpretation and development by the author. Oda often keeps details deliberately open-ended to keep the story intriguing.

If Horus had been victorious at Ullanor, what do you think his first action as Emperor of Mankind would have been?

 It is difficult to say with certainty what Horus's first action as Emperor of Mankind would have been if he had been victorious at Ullanor, as this is a fictional scenario and there are no historical records or accounts that provide insight into how Horus might have acted in this situation.

In the Warhammer 40,000 universe, Horus is depicted as a complex and multi-faceted character who is torn between his loyalty to the Emperor of Mankind and his own ambition and desire for power. After being named the Warmaster, the highest military rank in the Imperium of Man, Horus leads a successful campaign against the Orks on the planet Ullanor. However, he is eventually swayed by the influence of Chaos and leads a rebellion against the Emperor, culminating in the Horus Heresy.

If Horus had been victorious at Ullanor and had not been swayed by the influence of Chaos, it is possible that he might have continued to serve as the Emperor's loyal servant and Warmaster, leading the Imperium's military campaigns against its enemies. It is also possible that he might have sought to exert more control over the Imperium and to shape its direction and policies to suit his own goals and ambitions. Ultimately, it is impossible to know for sure what Horus's first action as Emperor of Mankind would have been, as this is a purely speculative scenario.