They're not my favourite, but I think we have to give credit to the S&S shot/drop towers. They've been a hugely popular model that have been installed in god knows how many parks, from huge parks and chains like Universal, Cedar Fair and Six Flags to crappy roadside attractions and everything in between.
As enthusiasts, we may be bored of seeing them - I only ride them these days if the park's quiet and I've done pretty much everything else - but they remain really popular with a lot of park guests, even though they're bound to have seen them elsewhere at this stage.
I'm struggling to think of a more successful flat ride type in terms of number of installations to be honest. Top Spins probably take a pretty far behind second place.
My personal favourites are Intamin drop towers, specifically Gyro Drops. Well, those that actually rotate anyway (most of them) on the climb up. UFO at Formosan Aboriginal Culture Village is probably the best of those thanks to the theming and views.
I tend to see things like Tower of Terror and Mystery Castle as more overall "experiences" than drop towers, otherwise they'd clearly be at the top of the list.