Everyone one will most likely say the best course was the one they did. I don't think there's anyone on here that has trained with every single company.
You should do a search on here on the different lash companies, do a bit of research into each company on the rest of the web, make a short list and contact each company whether that's email/phone.
Once you've made contact you will get a feel for the company. Your training will not be the last time you contact this company so you want to like the people you are dealing with and you will want quick replies.