The least reliable drives for me have been 2TB eco drives, but 2TB in general seems to have been a weird spot in terms of reliability. The 3TB Barracudas aren't stellar by any stretch of the imagination, but I bought a LOT of 2TB drives to use for backup systems and I think I've replaced something like 40% of them over the last three years. Many of them were sitting in USB or eSATA enclosures. When I compare that to the number of 1TB and only-slightly-newer 3TB drives I have out in the world, that's a jaw-dropping number.
I only have about 30 4TB drives, but not a single one of them has failed as of yet, let alone the mass failures I saw with Seagate and WD 2TB drives. I'm not prepared to say that smaller platters are actually better.