1/8 nitro buggies, preferably OFNA- fast, versatile, durable
1/10 monster trucks- nitro, such as T-Maxx, run anywhere fun.
1/18 Brushless, very small VERY VERY FAST, some get these upto 80MPH, mine does around 60 or more with outrunner power.

brushless is a new breed, and very fast, these cars must be monitored to make sure they dont grenade the trans/outdrives or over-heat, they are best run with lipo's which can be inexpensive at places like hobbypartz. the best thing with brushless is there are no comms, brushes or break in, and are more efficient. also, most ESC's are fully programmable and offer timing, volt cutoff, start up, brake/reverse options and more.

i still enjoy nitro, and bigger models are always more fun with them, 1/8 buggies are pure performance and the engines powering them can be in excess of 2.5hp and 40,000 RPM they are also the toughest things ive ever owned.

if you just want to play around, 1/10 monster trucks like the T-maxx and others are for you, i still like nitro in this criteria.