At the VERY minimum I'd say a Kraken if you have some mechanical aptitude and are willing to do some shimming and regressing. After that close it up and play with it until something fails and then do some upgrades/replacement of broken parts.
If you're a beginner who just wants something out of the box to play with, JG and CYMA are probably what you'll be aiming for but they do come a bit more expensive in the $300-350 price range. Then if you get AV'ed there's options for used rifles that are usually slightly upgraded or better compared to their NIB counterparts but also in the $300-450+ price range. Examples are my first rifle that I bought used for $450 (no mags).
If you have a bigger budget then ~$600 is a really good place to start at and you generally won't need to do anything to it unless it's a lemon (usually at this price range QC is higher and thus lemon rate is lower) and you won't need another rifle until you get the urge to rather than when it breaks down.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE