In part it is going to depend on your PBX version and where you want to go and what security/compliance requirements you have. Avaya's base IP Telephones are the 96x1 sets however you might want to look at video capable endpoints such as the H175 or the Avaya Vantage (coming soon). If security and survivability are not a concern look at the E129 or J129 SIP endpoints or even third party sip endpoints such as the Grandsteam GXP3275. Do you want SIP or H.323, Admin users, or Call Center agents, Bluetooth headsets, wired headsets, no headsets? Avaya publishes several comparison brochures you might want to look at. With Avaya's current direction you need to upgrade to 7.x or migrate to another manufacturer. If you have to meet DISA/FIPS/FISMA/PCI-DSSv3.2 you need to be on CM7.x and I would suggest the 96x1. The 4622 was typically a call center phone. The 9608 is the base model most customers seem to migrate over to. Stay away from one-X Agent since development is deprecated and there are no plans to support TLS1.2.