Thats a hard one for me. I prefer Spartans. Or rather Spartan II's (Over Spartan III's) because their extremely well trained, from a young age their trained in the best tactics of war, teamwork, educated with some of the most useful information, math, english etc and such battles as the Battle of Thermopylae.
Spartans are usually quiet to everyone besides themselves (Exept Kurt-051). It's common belief that if Spartans didn't have their iconic MJOLNIR armour they'd be useless. But Spartans did missions before they got this armour, and Master Chief has always told his Spartans to never rely on technology, rely on your training and instinct. "Using your eyes and far better than using your radar" is one such example.
Spartans are the best of the best of the best as it has been said. They give their lives for humanity and give hope to the people of the UNSC and strike fear in it enemies, whether its rebels or Covenant.
Just because I like Spartans more doesn't mean I don't like Elites however. I find Elites to be great. They have so much honour and courage and they're strong mentally and physcially. In the halo book Halo: Cole Protocol. Thel Vadumee I believe tried to take on a soldier he'd never seen before. It was a Spartan. When they went close range the Spartan had a combat knife and Thel knew this Soldier looked different from the others (He also had surprise that a human had the same strength as him). Covered in armour, stronger, faster and noticably far more well trained. Couldn't see his face so he said he was a "Demon" also he pretty much said "Lol look at that knife, we had knives like that in the medievil age"
Elites even took a good look on Humanity after years of warfare. Elites are a warrior race to the core. Humanity fighting and fighting and fighting nomatter how 'primitive' their technology was, and fighting a battle they couldn't possibly win impressed the elites. The humans fought the enemy that the elites saw as the best which was themselves. If humanity being inferia would fight even throwing the "smallest of themselves into their fire" without fear than the elites saw that humanity had earned the right to atleast have a chance of joining the Covenant. So Elites are awesome to me, and their bravdo is funny to watch. Such as them using so much to kill Spartans when they see humanity as being pathetic most of the time. I love elites, just love Spartans more.