Well, I am very fascinated with succulents from the family Aizoaceae, so I am bound to mention Mesa Garden. Their seed sales are about to close (or have already), but will resume in the first couple weeks in January. They have, by far, the most extensive seed list of any sites I have seen, though they mainly sell mesemb and cactus seeds. Also, you really need to scramble to get your order in when the new lists come out, or you run the risk of many seeds being out of stock. That being said, they are very friendly, and their customer service is excellent.

I've ordered from exotic-plants.de a few times now. I've found the quality to be very good even on the time sensitive seeds like pachypodium which are some of my favorites. I also find they usually give you more seeds than what you purchased which is always a nice surprise.

I've ordered some seeds from baobabs.com and they seemed fine but I've mainly ordered plants and cuttings from them.

The last one I've ordered seeds from is Arvice seeds which were on Amazon at the time. The seeds were all probably good but I drug my feet for a long time before I used them so I didn't get great results but still had lots of seedlings.

My most recent orders have been from a guy on Etsy in Serbia. I've probably made 3-4 orders with him. It takes a couple of weeks to get, but he has a decent selection and prices for seeds. Also my germination rates have been decent. https://www.etsy.com/shop/UnusualSeeds?ref=s2-header-shopname

I haven't ordered from the Cactus & Succulent Society of America yet, but have seen excellent reviews. The seeds are donated by society members and are really cheap ($1 per packet), but you do have to be a member to order (and membership isn't free). Also, they do ship internationally.

I have awesome expirience with www.rareplant.me