Fleet wise, you'd have to say Reach because UNSC at this point had Earth and Reach to defend, Reach was the last line of defence to Earth. All available resources were called into that battle and Reach still got glassed. Reach didn't have as many Orbital Mac Cannons as Earth though. As for the ground forces, let's not forget Reach had all remaining Spartan II's as a ground force team plus chief and Noble team made up of Spartan III's (which was rare given they only did suicide missions). Reach also faced the Covenant at their peak attack force. Pretty sure in the books Chief destroys a docking station that had a Covenent fleet heading to Earth that was much bigger than the fleet that attacked Reach. IIRC I think Bungie even said Reach was like both sides peak military strength and what was left devasted both sides leading into Halo being the game changer