1) Intamin ones like Apocalypse and Fabbri ones like Detanator are generally thought to be the best and usually offer intense drops. There is a company (which i forget the name of) that makes drop towers where as soon as you get to the top you drop, which people say are also very good, but they are only in the US from what i know.
Other common drop tower style rides are S&S space shots (launch up), Turbo drop (launch down, but not as intense as detonator) and combo towers (they do both). An example of a space shot would be Ice Blast at BPP. I personally, don't think these are as good but you may think differently. Tower of Terror at Disney Parks and Mystery Castle at Phantasialand are enclosed drop towers, that offer longer rides and different experiences.
2) I wouldn't say that Apocalypse's drop (sit-down mode) is quite as intense at Detonator's, but it is still very good, plus it is higher and the views are better.
3) The sit-down side is standard drop tower experience, but still very good. I would say that the standard up with floor gives the most accurate experience of what standing up is like on a drop tower, but the stand-up without floor i found the scariest, because you tilt forward. The stand-up ones are scarier, but the drops feel very different to when you are sitting down. Apocalypse is probably my favourite drop tower (not counting Tower of Terror), because it offers so many different experiences and it is fun to try each one. Also, the queues are usually very short and i got 20 rides in on my last visit.
Note: the floorless stand-up is a little uncomfortable when you hit the brakes
For your last question:
Oakwood: Not got much, but is a nice park to relax at.
Drayton Manor: The park is pretty yuck, but they got more rides (not many good ones)
Megaphobia: Quite rough, but plenty of airtime and overall a really good ride that you should get lots of rides on as the park is usually quiet
Speed: It is a vile piece of ****. Worse than Saw.
Apocalypse: As i said above it is awesome and one of only two really good rides at DM (the other being stormforce).
Shockwave: It was pretty horrible when i rode it earlier this year, so don't expect a fun ride.
I prefer Oakwood, but i will let you judge where you want to go.