The best replacement for mechanize is watir.

The best alternatives to mechanize are rated as following:
★★★★★  watir
★★★★  scrapy
★★★★  requests
★★★  selenium python
★★★  lwp
★★★  capybara
★★★  nokogiri
★★  beautifulsoup
★★  splinter

Your alternative for mechanize is not listed? Suggest!
Speaking about mechanize, one probably shouldn't forget about nokogiri.

Other search terms related to mechanize and their ratings are:
★★★★★ nokogiri
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mechanize
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