Favorite? Tough question. I can't really say I have a favorite. That's why I have different scopes on different rifles. They all serve different purposes.
I recently placed a 2.5-16x40 Bushnell 6500 on my calling 5.56. I use it for hunting and plinking. I really like the low-end power and the high end for paper punching and small targets way out there. The glass is pretty awesome too. I also have a red dot and magnifier I can slap on at any time if I plan on doing some heavy cover, thick woods hunting.
My AP4 .308 wears a Leupy VX3 Illuminated 1.5-5. I use this for deer drives and heavy cover hunting. Hopefully, I will be using it for wolves here in Wisconsin soon!!! I also have a Zeiss 3-9x40 for it that again, I can swap out if I'm sitting in a stand and shots are longer.
My Bushy Varminter wears a 6-20x50 Burris. This gun doesn't see much hunting use due to the length and weight, so as a dedicated pdog and bench gun, this scope is almost a perfect fit. The scope on this gun is in Bobro mounts so that I can swap it to other guns for load development. My heavy barreled Rem 700 .22-250 wears a 6-20x50 Leupy VX3. This is also a dedicated bench and pdog gun (weighs about 13lbs).
My .243 WSSM wears a Weaver Super Slam 3-15x50 w/EBX reticle. With the wide zoom range, this scope gives me the low power for close stuff, and the high zoom for the longer stuff that the .243 can handle.
In order to find out the perfect scope, you really have to sit down and look at the task at hand for your rifle. None of the scopes above are perfect. If I had a few grand too spend on glass for each, then I'm sure I could find the perfect scope for each. What I did was pick the scopes that fit my task for each rifle and that fell in my budget at the time. So far, it's working for me.
Another side note. If you have a rifle that is used for many different purposes, then it might be helpful to have your scopes in high quality QD mounts (LaRue, Bobro, & ADM). I started doing this as it is cheaper to invest money into mounts rather than a new rifle (although new rifles are cool too!).