I'm going to focus on the Dougal Disk here, as it's this that I purchased the cd for. Well it's been a few years since Bonkers 13 was released, and I still find that the Dougal CD is the best hardcore CD I've heard to this day. The Brisk and Kevin Energy CDs on Hardcore Heaven 1 (the one with cartoon artwork of a wierd helmet) are on par in my opinion.
The CD really gets you in a euphoric state of mind, a hardcore paradise if you will. The aptly named "Heaven on Earth" is my favourite track on a mix that builds up beautifully. I think the tune selection is great, they all have mad, clean sounding euphoric riffs, along with some screeching synths in "Xtreme" and "Get Hype." After the insane beginning the mix goes into a brief lull, "Right Here", "Still the One" and "The Theme" are not my favourite tunes. Yet it soon picks up again with some more pounding euphoric hardcore, and as you enter the later stages of the mix you get some really emotional riffs in "Night Flight", "Close Your Eyes" (which begins with an awesome pounding hardstyle bassline) and "Substance". These three tunes, in my mind, really reflect the later stages of a messy night out drawing to a close. "Drifting Away", the last tune is again not one of my favourites, but it's ok and I know a lot of hardcore heads love it.
All in all I'd say if you were new to hardcore, or trying to get your mates into hardcore, then this is definitely the cd to start with. I'd reccoment buying the compilation just for this cd. If you like getting messy at home with friends, girlfriends or whatever, then wack this on for an awesome experience that would even rival being at a rave.
As for the other CD's I cannot say as I've hardly listened to them. I'm not a fan of Old School which is why I've not bothered with Hixxy. Scott Brown is one of my least favourite hardcore DJs while the freeform sound of Sharky I only like on occasion.