Actually no specific answer for that, is depend on a few creteria such as:
1- Features that NE(MSC/VLR, HLR etc) support(although all vendors claim that their follow the standards body.)
3-O&M capabilities ( it will easier for O&M guys after)
4-Also easy of conversation while any problems related to NE. For example if you choose the vendor difficult to convers and talk, then O&M will be difficult after.
5- Last this is most important and sometime we from low level cannot avoid that. 🙂 the higher management decision. The decision sometime not follow the above criteria, but of cost the value/price and also the power of give and take(you understand me..;)
From my exprience, Nokia is the best, expecially interms O&M capabilities and features. Ericsson, alcatel. Siemens good interms of GMSC and International Switch. Huawei ?!!! the chinees vendor good in price but, i’m not sure interms of capabilities and features..
But want i can suggest to you, do the network trial ( you can put it as a request while doing the RFP). The trial between different vendors with all capabilities and features need by your network. Then from that, you evaluate the best switch for your network, But point 5 come in mine…