The best site showing flat rides is the Flat joint which can be found at:
http://www.flatrides.com/
This is the best way to see any flat ride you want to see, as it has a huge ride index with most of the flat rides ever created. It is really hard to pick my favourite but i think it would have to be made by either modial/KMG/Huss! My favourites are (in order)
1) Mondial topscans - look amazing - great idea!!!! great ride!!!!
2) Huss Topspin - feel amazing and very fun to watch. Especially ones with working water fountains - e.g Ripsaw at Alton Towers.
3) S&S sports space shot - would you count this as a flat ride? I wasnt sure, but you get a great feeling when you suddenly shoot skywards, and then you get great airtime and views at the top!!!
4) KMG Afterburner - amazing feeling when going at full speed and swing. You get some great G's aswell sometimes.
5) Huss Fissbee - Same as KMG Afterburner, although the huss ones can spin faster. Well it feels like they do but often dont swing as high. Although the giant Frissbees look great, but ive never ridden one and never spoken to anyone who has. This is the reason why The Huss one is after KMG's, but the Giant Huss one would probably win.
Thats my top five. I also rode a decent shot and drop tower at Lincoln (UK) Christmas Market, although when we were a the top i thought it was gonna break. lol