Are you trolling? Market square is fantastic. And forager has lots of applications too, as non-terminal +buy and trashing are fairly rare. I'd say it finds a frequent home in engines. And if there are alternate treasures, and/or incentives to trash treasure (remodel, contraband), look out, it just gets silly.
No I'm serious here. Market square is good in engines, but useless in Big money, slogs or rush strategies. And forager is usually used for trashing curses, estates or coppers right? For coppers it's just a +1 action, which is worse then loan. It provides money for curses and estates, but most of the time only $1. Loan can't be drawn dead, while this can. Forager will become useless after estates and coppers have been trashed, while loan will provide money still.
Cantrip+buy is pretty decent already - not exactly a good $3, but probably pretty comparable with some of the other $3's here on it's own. As for the gain effect, it's awesome in BM, very good in most engines (especially, early on it gets your Golds without you needing to buy any, and after you have as many as you want, you can probably utilise the extra buy more), and it's not a bad thing in most other decks. In BM, all it needs is some kind of light trashing - any kind, even upgraders works, and suddenly you have something that can provide you with Golds ~50% of the time you see it in your deck, and is a cantrip if you can't. And some kind of light trashing is pretty common, so that's a good thing. In engines of course you can hit and activate it even more consistently. And it's very good against trashing attacks, of course.
Tunnel's 2 VPs is probably better, unless you need an extra buy, I won't deny that, but Tunnel is also much harder to activate, so many more cards will activate MS reliably, and that's ultimately the main purpose of Tunnel.
Really the key here is, Market Square will be getting you golds probably 50%+ of the time it's on the board, while Tunnel is considerably less.
Forager is pretty good in my limited experience - not great, but decent, above this section. It doesn't give you the extra coin of Loan, true, but then after you've trashed one copper, it trashes Estates and the like for +$1 and a buy, which is nice. It also has the potential to get even stronger in games with trashing attacks, or otherwise the desire to trash silvers. Of course these benefits also apply to your opponents, so caution is needed.