What a great question. Good thing I don't regret the decisions. All the time I spend studying a deck, even if it doesn't work out, contributes to my overall understanding of tarot as an art form and as a spiritual tool.
Mine would be:
Fey Tarot
Shining Tribe Tarot
Osho Zen
Sweet Twilight
Rider Waite Smith
I would add Haindl but I didn't spend much time studying it. When I saw the Kali card it just completely turned me off the deck.
And, goodness, Thoth was my only deck for 15 years. I studied it along with Crowley's entire canon and really got into it. Even after years of study it proved bafflingly obtuse when it came to reading. I still have two Thoth's and love to look at them but they rarely come out for a reading.