which of the following friendships was stronger?

Legolas and Gimli, hands down. Their friendship was renowned as "greater than any that has been between Elf and Dwarf." They stood shoulder-to-shoulder during the War, as companions and equals. They had each others' backs. Both were willing to step way out of their own comfort zones for the sake of the other, as shown by Legolas' visit to the Glittering Caves and Gimli's visit to Fangorn. They were so close that when Aragorn died and Legolas sailed to the Undying Lands, Gimli went with him.

The notion that Aragorn and Legolas were great friends of long acquaintance is Peter Jackson's invention. No doubt they became friends during the Quest. However, in the books, there's no indication that they knew each other well from the start; it seems likely to me that they met when Aragorn delivered Gollum to the Wood-elves, but not before. Aragorn journeyed to Rohan, Gondor, and even Far Harad, but there's no suggestion that he spent time in or near Mirkwood. In Eriador he spent time guarding the Shire and Bree, and he had responsibilities as Chieftain of the Dunedain. Legolas for his part was not well traveled. So there is not much chance that they formed a close acquaintance prior to the Council of Elrond. And while they became friends, Aragorn and Legolas did not become buddies in the way that Legolas and Gimli did.