I'd probably give it to Luffy but I don't recall if we ever saw how strong his base level was immediately after the time skip, at least without Haki. Hard to say.
I don't see why he wouldn't get stronger in general. Pre timeskip his base level powered up whenever he went through something difficult, I don't see why that would change in regards to 2 years of difficult training.
Yeah maybe not. It took all of the Straw Hats to beat a pacifista pre timeskip and Luffy was able to beat one with G2 post timeskip. He clearly made big gains but it might not be enough to beat Lucci.
For the most part he learned to master his gears and his Haki. Any gains in muscle strength was likely a side effect of the giant beasts he had to deal with on that island, so not nearly as great I would think.
But where do we draw the line? Because if Luffy is now Yonko level, how could his Base still be so weak in general? He hasn't been training his regular Base form but it would be absurd if he could lose to the likes of Caribou for example.