That's really a tough choice between Call of Duty and Halo.
AAA: CoD, CoD2, CoD4, Halo, Halo 2, Halo 3
AA: CoDWaW, Killzone, Killzone 2
A: CoD3
IMO, so I would give the edge to CoD just because there are more games available. Halo follows super closely and then Killzone with a really big distance.
1. Call of Duty
2. Halo
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3. Killzone