ViaNet is the best choice if you are inside the Kathmandu valley. Outside the valley Worldlink is the best, I guess. I have been using Worldlink for 6 months and it's quite impressive ,it's speed and maintenance. I have a package of 5Mbps(average 600KBps of download speed but I get 1.1MBps while on latest quantum firefox). I don't know about Classic Tech but I heard that it's maintenance is not quite good(or maybe rumors). Well, there are other ISP's named Broadlink and Subisu which are quite good in speed (I was suggested by someone) but I doubt on their maintenance.
Personally, I recommend ViaNet or Worldlink. The choice is yours.