For me, I am struggling to decide between halo 2 and halo reach as they both have really good art styles and character designs, but for weapons I would probably say halo 2!
For example, nothing says halo 2 quite like the beam rifle, the covenant carbine, the beautiful energy sword, and the battle rifle. Something about those to me just... I dunno! I really like those weapons a lot. From reach, I really like the needle rifle a lot. Definitely a go-to weapon imo.
With reach and its art style, it introduced the skirmishers for the first time, and I love 'em! The armour and looks of the elites, the looks of the jackals, and the armour of the grunts are all pretty cool too. The only thing I'm not too keen on is the design of the brutes - halo 2 did that waaaay better in my opinion. 
I will say though, halo 3 has got some amazing looks to the game still to this day. Of course, there are some things that haven't aged well (*cough* lord hood *cough*), but the lighting, weapon design, sounds, and overall feel is really good!
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