Hi Alexis,
"Best" is pretty fungible—depends on what you're looking for. I'll also just briefly note that as far as SFDC integration goes, your best bet is simply to populate your deliverability fields with SFDC-synced fields, and obviously you can do that with all three services. I've used all three in the past, so:
Informatica: I actually quite like Informatica's set up with Marketo for real-time validation, though in terms of price point I'm not sure it's the best value. Moreover, I'm a little distrustful of its flagging of honeypots—I've seen a couple of false positives in very large datasets, but it's accurate for most marketers' needs. It's very "set it and forget it" if you're not the type to geek out about making delivery lists, which is a big perk to some. Now, when you ask about robust reporting, I do wonder how deep you want to go down the deliverability rabbit hole, but Informatica is pretty middle-of-the-road there. They mostly just want to group by labels rather than provide more sophisticated scores.
Kickbox: Normally, I don't use Kickbox's integration with Marketo (I prefer to use Kickbox for bulk projects along with some proprietary methods for real-time validation) but I've seen similar success to Informatica with their connector. The return data is more detailed (Sendex etc.) but it can be harder to make discernible calls on what to do with that data if you're not as versed in the differences of "accept all with a Sendex of 0.65" vs. "accept all but role email." Also one of the more reasonably priced/accurate ratio services right now.
Mailgun: I've never used Mailgun specifically for email validation (normally it's just bolted on for transactional email and thus out of the Marketo ecosystem) but Flanker is open-source if you want to go down that route. Honestly, it's more similar to mailcheck.js than the first two services (as it's looking at formatting and rules rather than actually pinging the address), but I'm always an advocate of using Mailcheck with something like Informatica/Kickbox, so if that's the route you want to go down, there's no harm there.
.png)