Either way, Vader is stronger than Galen Marek.
Yes, Vader was defeated by Galen Marek in the original Force Unleashed novel. However, in the heat of the battle itself neither gained any sort of significant advantage, the novel even pointed out that they faced one another as equals. Marek gained the edge by baiting Vader with a tactic reminiscent of Dun Moch, and pouncing when Vader's anger made him misstep.
However, in the Force Unleashed 2, the case was not the same. Yes, similarly to the first battle, Starkiller and Vader had fought to something of a stalemate. There is a difference, though, and that difference is that the clone of Starkiller has more impressive feats than the original in the first place, thus Vader likely had improved between the two works.
Then we come to the original trilogy. While it may have been arguable before that Vader had improved between the two Force Unleashed games, it is not arguable that he improved significantly over the course of the OT. In Empire Strikes Back, the Fightsaber articles suggest that he had trained relentlessly since ANH, and thus had become a far more formidable opponent. In Return of the Jedi, Vader muses that he was currently more powerful than he had ever been, supporting his consistent improvement.
In the end, RotJ Vader is superior to Vader as he appeared in the Force Unleashed series, where he was depicted as Galen Marek's equal. /thread