I am looking at some cars and they say some fractions. I know it is a noob question but I always just got what was fastest for a cheap price.

Scale size of model to actual size of vehicle.

what size is best for speed and agility combined?

1/8th scale is a sort of plateau for speed, both in RTR and to a degree in race rigs. For example, the Traxxas Slash 4x4 VXL is rated at 60mph for 1/10 scale, but the 1/5(?) scale X-Maxx is only rated for 50mph. Weight factors a lot. 1/10 is the most common.

so would the Quanum Vandal 1/10 4WD Electric Racing Buggy be a good size for some one who has never used buggy before

The slash is a short course truck and the X-maxx is a monster truck... their speed rating has nothing to do with scale. The only limiting factors to all out speed in the RC hobby is money and operator skill/knowledge.
No reason you couldn't make an x-maxx do 60mph if you really wanted to. Why would you, though?



@Offroadrcracer

"Best" is incredibly subjective, so what you really gotta do is tell us what kind of surfaces you are going to run the car on. That will get you the most useful answers. "fastest and most agile" can mean anything. Maybe a buggy is not the right thing for the surface you run on. My RC10 B6.2 is very agile, but there isn't much that can be done to make it go anywhere at all on grass (unless its a golf course fairway).

Also, what do you consider fast? Some consider 35mph fast.

Based on my personal experience with the Quanum Vandal, I would not recommend it as your first buggy. It's poorly made and put together and the buggy could not handle the power plant. Mine lasted about 2 minutes of backyard bashing before multiple screws coming loose and the front a arm breaking. You're better off buying a nice quality preowned brand that's going to stand up better to the abuse.

I consider really fast 60-70 mph fast is 45-50 and 35-40 is acceptable 15 is unacceptable

RTRC is correct on this.
Just by looking at it will show some that reliability will not be be good. It has a fast motor, but the buggy is made of too much plastic to handle that speed. Even the shock bodies are plastic. Also, the bottom of the chassis is open right under the spur gear. This will allow a ton of debris to get caught up in the gears and strip them out. It might be pretty inexpensive, but I promise you that you will spend at least $100 every couple months to make repairs. If you are new to the hobby, it's best to go the biggest hobby shop in your area and look at what they have. You can come back here and let us know what they have, and we can help you make a decision.

Is that on tarmac, or dirt?

60-70mph on pavement is not hard in a 1/8 e buggy on 6s. Same speed on dirt / thin grass takes Overkill level of power, probably on a 1/8 truggy so you have lots of suspension...

A few RTR ideas / examples,
70mph+ on tarmac, running less than one minute, then letting the car cool off - Arrma Typhon 1/8, Talion 1/8 or Infraction 1/7
60mph on tarmac - Arrma Typhon 1/8, Talion 1/8 or Infraction 1/7
50mph on dirt/grass - Kraton 1/8 /Typhon (on 6s) (most any 1/8 truggy on 6s)
40mph on dirt/grass - Slash 1/10 / 1/10 Senton (many SCT on 3s) (most any 1/8 buggy on 4s)
30mph on dirt/grass - 1/10MT like Granite, Hoss
<20mph - any brushed model

The dirt speeds are not the speed listed in 6" tall letter on the box (those speeds are mostly unrealistic except on pavement, single pass), but rather what you can really expect the truck to do out of the box or with minor tuning (pinion swap) without overheating the motor every battery. Also, lots of other cars could be put on that list (they could, this is just examples of a truck class), and some will argue the cars I did put on the list should move up or down a spot or two. But, this is my list. And yes, the list is Arrma heavy, because i have a lot of Arrma trucks, and know the line well. Lots of other brands can go fast too.

Smaller than 1/10 scale can be fast for cheap, but they often don't have the weight / ground clearance / suspension travel to go fast on anything but pavement.

With that said, a 1/14 truck with a 25mph top speed can feel very fast and fun in a smaller running area, where a 60mph 1/8 truggy would be very boring, because you would not have the room to get it up to speed or use most of it's massive power. Make sure you get the right size RC for your preferred running area.
1/8 - BIG parks, wide open spaces, no people (50mph 10lb truck can break bones of that kid you didn't see)
1/10 - bigger yards, medium size parks, front yard & street (depending on traffic)
1/12 and smaller - small parks, back yards, small dirt lots, big basement / living rooms/garages

YMMV
Jerry