Depends, what equipment is being replaced? In the big picture there isn't any one vendor/brand that has the "best" of everything. Nortel has crossed over to data with buyouts/mergers with other companies. Cisco has done the same with voice/telco areas.
Cisco isn't the only big name in routers but in my experience the best I have worked with. We have Nortel phone switches but in VoIP Nortel is lacking. It really depends on what you are trying to accomplish.
Cheers,
Mark ;-)