The best replacement for espeak is festival.

The best alternatives to espeak are rated as following:
★★★★★  festival
★★★★★  pico
★  flite

Your alternative for espeak is not listed? Suggest!
Speaking about espeak, one probably shouldn't forget about mbrola voices.

Other search terms related to espeak and their ratings are:
★★★★★ mbrola voices
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Google Trends

Scale is based on the average worldwide traffic of espeak in all years. Click on graph for more.
espeak
festival
pico