My top five, in no particular order:
1. Helioskrill. As someone who loves the Sangheili and always has since Halo 2, the idea of them making Mjolnir for Spartans is nothing short of awesome.
2. EVA. I love all forms of EVA, be it Halo 3, Halo: Reach and yes even GEN2 forms. I remember being so happy finally unlocking EVA in Halo 3 after hours of trial and error. When I was a little older and I got to watching RvB, The Meta made that armor set twice as awesome. It'll always hold a special place in my heart.
3. Mjolnir Mark VI. There's not much to say about this one. It's the perfect blend of old-school classic armor and just a really simple, well-working design overall. it just works.
4. CQB. This one has a lot to do with my brother. My little brother and I used to play tons of Halo when we were both wee. From Halo CE to Halo 5, we grew up together as Spartans. His armor of choice was usually the CQB when applicable, and anytime I see it I think of him and the great times we had in Halo 3 and Reach. I also just love the t-shape of the visor, it subtly reminds me of the Star Wars Mandalorians.
5. Thinking of a fifth was a little challenging, not because I don't like any other armors but because I like all of them. But I'd probably have to go with Emile's modified Mark V EVA armor, with his signature kukri and carved death's head. Emile himself shares my birthday, and on top of that, the kukri reminds me of my father who also plays Halo and the armor is just badass overall, not to mention it's a one-up from my other favorite, the EVA.
That's about it, though if I had a say in the matter, I would prefer to play as a Sangheili!