I love the AR15 platform. Sweet as pie.
I currently have 2 models of the AR15. I have a Colt MT7600 that was built in 1998 during the so called "assault weapons ban" (that was nothing but a feel-good law). It has been properly modified to be a civilian M16A4 (threaded barrel for FH and milled barrel down from HBAR to govt. profile). Currently, it has Burris MTAC 1-4X mounted on top. It has the 1/9 twist so it fires both the 55gr and 62gr bullet equally well. I've taken that rifle with both iron sights and an ACOG through multiple rifle courses. Probably my "go to" rifle in case the zombies ever come. Invested price tag with gunsmith mods: about $850 circa 2005.
My 2nd AR15 is a Frankenstein M4-gery, meaning it is made up of various manufacturers and it as close a civilian can get to an M4 without proper licensing (it is missing the happy switch and it has a 16in barrel instead of a 14.5in). Currently, it does not have optics mounted. This too has the 1/9 twist. It is light and compact (comparatively). Invested price tag: less than $500 circa 2015.
As GMAN pointed out, read up and PRACTICE with your rifle. The AR15 is an awesome platform and it is unsurpassed when it comes to accuracy, ergonomics, and reliability. However, being a highly engineered piece of equipment you need to maintain it. Oil it (NO GREASE!!!), then oil it some more especially in the metal on metal parts within the action. Keeping it wet is the key to keeping it working between cleanings. Ask any veteran who used their M16 or M4 on a regular basis in Iraq or Afghanistan and they will vouch for keeping your AR wet. Some of the benchrest aficionados will tell you in a snobbish manner that they use "band X grease". Let them. The M16 was designed to be lubed with oil. I suggest Breakfree CLP, or something similar. Do a search. But whatever you do just keep it wet. Oh.. and make sure you PRACTICE PRACTICE PRACTICE!!!
My third "fun gun" is a Vector V93A3 (an HK93 clone). Though not as shooter friendly as the AR series, it has its benefits.
With the telescoping stock, it becomes very compact. It is heavier and a bit easier to fire accurate with continuous fire vs the AR, but a rifleman doesn't need rapid fire, does he.
Eventually, I may invest in the Israeli bullpup Tavor but who knows. The Sig 556 series really catches my eye, too.