This question gets asked a lot and you'll get all kinds of responses (the old "opinions are like bungholes..." bit). You'll get better feedback if you tell us what kind of budget you have, what equipment you already have (marker, air/CO2) and where/how you play on the field (back, front, bunkermonkey).
To summarize some of the previous discussions on this:
- look at a second-hand marker. you'll get more for your money than buying new.
- mid-range electros are a good option. opinions on low-end electros vary.
- Autocockers are also a good option and give you room to build/customize your marker.
- see if you can try out different markers at your local field to get a feel for them. most players will let you take a few shots at the chrony if you ask nicely.