People of course have varying opinions on the "perfect" card stock, since it often depends on the size of the cards, as well as the size of a person's hands and hand strength - and how much they like to shuffle their Tarot cards. Personally, I prefer thicker card stock that's built to last, but on the other hand I've never felt the need to riffle-shuffle my Tarot cards like I would a regular deck of playing cards. I prefer to take a little longer mixing them well, with mostly overhand and hindu shuffling, and maybe fanning them out once or twice, rather than stress the cards with a lot of riffle shuffling to get it done faster. Well, okay, sometimes I do a little "light" riffle shuffling, but definitely no bridging/cascade finish. So...although I prefer the thicker stock, at least I know if I get the thin stuff the cards will probably hold up for me longer than they would for a lot of people.
Lately I've been using the large Tarot of the Holy Light deck - 3.2 X 5 inches with 350 gsm card stock and a linen finish - and I'm perfectly happy with the size and quality. Maybe not the thickest Tarot card stock out there, but thick enough for me, and I wouldn't want them any thinner. And I think the slightly textured feel of the linen finish makes them easier to shuffle (however you like to shuffle) - although you might want smaller cards if you have small hands.
It isn't ever just a question of how thick the cards are, though. You have to take into consideration the quality of the paper, whether it's core-less or black-core or blue-core or whatever. I don't pretend to be very knowledgeable on all of that, although I gather black-core stock is generally considered the best. But that's not the kind of thing Tarot deck publishers usually advertise (not that I've noticed), so you'd probably have to ask them if that kind of thing is important to you.
