I would suggest something that balances well.
There is nothing harder to use than a badly balanced gun, Irrespective of weight.
M4/AR15 there's a load of specna arms guns out there that will work very well.
My personal gripe with the M4/AR15 platform is the tacticool looks and balance. The lightweight crane stocks and heavy rail sections on some models can lead to very unwieldy guns. The VFC HK 416 is a prime example of this, Lovely gun but the rail is so heavy and not balanced because you can't get a good-sized battery in it to counterweight it.
The best M4/AR15 platforms for balance are the ones with solid stocks, where you can use a larger battery (personal opinion), For this reason, I would rate the M16 series of guns that are supposed to have solid stocks as being better.
The Classic army M15A1 Vietnam (actually an M16A1) has a lot of features, Is mostly metal and has a MOSFET installed. For £200. It's well balanced and shoots well out of the box. And you can balance it to perfection if needed as there is space at the front and rear for lead weights.
https://extremeairsoft.co.uk/shop/assault-rifles/electric-aeg-rifles/classic-guns/classic-army-m15a1-vietnam-x-series-rifle/
The Aug platform that
@Asomodai recommends is also a gun I would recommend, however you have to want that platform. If you have no basic interest in it avoid it. Augs are different, they don't follow the same internal components rules as M4's, and have a habit of getting BB's stuck in the body. Not an issue if you like the gun (I own a few) and love them. If you just want a simple no fuss gun it'll frustrate you.
Same with the P90, Mag compatibility can be really bad at times, and the internals are a different configuration to M4 platforms.