If you want a fun cool-guy-gun and don't mind burning the cash and dont want an AR-15. Go Scar 16.
It has the added bonus of being a battle tested top quality rifle that you could trust your life on. The reciprocating handle is frankly a non-issue (think AK's). It's very light, soft shooting, and accurate. Yes, it is overpriced, but it is also a really good rifle. Many upgrades if you want them.
-Bren 805's awesome, basically heaver SCAR's with excellent stock triggers, but you will always be hankering for a BREN 2
-Tavors are fun, until you get a malfunction/jam and start questioning your sanity and your life choices. On the other hand it's probably the best bullpup out there and looks like it is "Space Force" ready. Afterall "the only good bug, is a dead bug" Do you want to know more? With Geiselle upgrades, they are pretty fun.
-ACR's are ok, but the quality control is suspect, but the platform seems to be finally expanding (just in time for Bushmaster)Cerberus group to go bankrupt so factory support might be questionable). Also they inexplicably weigh a million pounds (the barrel/bolt carrier).
Its a super soft shooter though.
-Sig MCX Virtus. Cool gun but some big question marks: did you get a good one (thanks SIG QC)? Will they generationally advance next year and not have support or backwards compatibility?
It's heavy! between the barrel profile and the giant thick hand guard, this thing is maybe more nose heavy than the ACR. This is solved with a nice light MI handgaurd, other recommended upgrades include a trigger, and charging handle. Also it doesn't include iron sights in the box (which is fine), but you'll need those to shoot it out of the box OR just an optic because it's 2018 and do you really need irons?
If you have to massify, you lose alot of the benefit of the MCX over an AR, however the barrel/caliber changing is pretty sweet.
-Galil ACE. Looks ok, but I am not sure about it's track record. I have a hard time trusting a rifle that has never been adopted buy any military. I haven't owned or shot one so I dont have a strong opinion. I have heard good things tho.
If you want the best possible rifle with the most amount of customization get a real deal preban Colt and buy a new upper or three and call it a day. In the end it will will probably cost you the same or more as a new gun. It's pretty hard to beat an AR-15.
For the last 30 years the US military has been trying to replace the AR/m16/m4, there is a reason why it's still around.
Hint: because it is still the best.