What are today's best Web project development software environment for us to consider now?

Back story:  We have a really big VB6 application that we initially developed over 15 years ago and continue to enhance for our users/clients in many fortune 1000 companies.    SQL server backend for the data tables.   We know we need to move away from the legacy fat client application to a web platform for clients to use in our cloud or within their own secure networks on Web servers such as IIS.

We need to choose the conversion's  Web developer environment wisely.   The rewrite of the current VB6 application we know full well is a going to be a huge undertaking in time and money.   We have a fixed budget for this project.   We want to make the right choices now.

ASP .Net may belong on our short list since some of the coding and coding logic may be partially portable.     Lots of good choices may include other environments such as PHP, Java, Perl, Python, C++, others, etc.     Interesting list of the programming languages used by the biggest websites here.

Please share your thoughts and experiences, thanks!