Depends on what you mean by "best." 
There are many options:
- A Google Home Mini (or any similar device) with the Hubitat-native Chromecast (beta) integration. Forum thread: Google Home Mini - A great little tts speaker
- Similar to the above but super-unofficial is the Alexa - -Unsolicited TTS and [Release] Amazon Alexa Text to Speech (TTS) v0.5.1 - Direct Integration (USA, Canada, UK, & Italy) community apps = (haven't actually tried this myself and there hasn't been much discussion on it lately, but I think it still works, though it has always required some babysitting)
- Sonos with the Hubitat-native integration (probably does not meet your definition of "cost-effective" unless you already have one...)
- VLCThing, if anyone has managed to successfully port or re-write it and you have a Pi or similar to run the "server" on
- LANnouncer, but I'm not sure if the maintainer has officially ported it, with a compatible tablet like any Android running the LANnouncer app (Amazon Fire tablets are a common, cheap-ish option and many people already have these in use for dashboards)....but it can be a bit delayed if the tablet is sleeping (and you need to make sure it always starts on a reboot).
Personally, I'm using LANnouncer for the rare instance where I want TTS. It works pretty well aside from the minor tablet-sleeping issues (I think Fully Kiosk Browser, also used by many people for dashboards, now has support for TTS, so it might work better than LANnouncer since if the tablet is always on, the app will likely always be in the foreground, but I haven't tried this and I'm not sure if anyone has written Hubitat code for this yet). If I weren't already deep in the Amazon ecosystem, I'd consider the Google Home family of products (but if you find one for cheap you could always just get one anyway and mute the mic--I've heard they're good even if announcements and whantot is all you use them for). And if I didn't think it were a lot more money than I'd want to spend, I'd definitely do Sonos.
...and there are probably even more options that I don't know about. These are just ones I'm aware of because I've considered most.
