Best flat ride in Vegas - X-Scream at the Stratosphere. There is nothing like a see-saw... 1,148 feet in the air.
Insanity is also a great flat atop Stratosphere, completely suspending riders over the edge. I would recommend skipping their S&S skyshot; it has been drastically neutered over the years and pails in comparison to other S&S shot towers, even given its height.
For roller coasters, with Speed still in pieces behind Circus Circus, Canyon Blaster in the Adventuredome takes the cake. It is actually a very smooth Arrow Double loop/corkscrew, with a fantastic layout thanks to the fake rock work inside Adventuredome.
Roller Coaster at New York, New York is worth riding to say that you had ridden it... and that is it.
Honestly though, I'd recommend skipping the rides and putting your budget towards a Cirque du Soleil show. If anything, all the rides are gimmicky, and are very mediocre compared to regular amusement thrill rides.