This would be my list in no particular order:
Aastra
Polycom
Snom
Linksys
Cisco
Mitel (Dual Mode)
Yealink
There are a lot of nice xml scripts available for Aastra phones. I would also look at EHS capability, headsets. If they are EHS compatible, no handset lifter is necessary. Are you going to be using an endpoint manager? Check which phones are supported. Cisco phones are great, but a pain to configure, (my opinion). Polycom's are easier, but also want to reboot with the smallest change. Mitel's are great quality, but you have to convert them to sip. Yealink, have only used in one install. Linksys and Aastra, both good phones, but seems as if there has been more "done" with the Aastra/Asterisk relationship, i.e. xml scripts.