if you go to in and out, have them say "add chilis" they will cook your patty with the banana pepper. it is much better than adding them afterwards. i also recommend saying "no salt" they often heavy hand the seasoning while cooking, and especially if you get it animal style, you cannot taste the difference.
holsteins in the cosmopolitan, center strip. has really good burgers. the meat quality is very nice. i have had all of them.
i know you said that going off strip is not really an option but Bachi burger, south of mandalay off Lv Blvd about 8minutes has really good asian inspired burgers. again very high meat quality.
burger bar is pretty good, i find you need to be careful on the build your burgers though. most people just pick things they like, but dont consider how the flavors and textures play off of each other and end up with a burger they are content with but feel they could have done better themselves, or gained more value elsewhere.
stripburger is barely ok to me. they do have some good pairings, but i have always felt unsatisfied. i used to work at nordstrom and they had generous mall discounts, so i ate there a good amount of times. id much rather in n out.
each hotel does have a fancy burger place and they are all pretty good, there are two i have yet to try, one in mirage and one in the palazzo, but ive yet to have better than holsteins on the strip. off the strip ive found some good local joints for sitdown burgers
for fast food, in n out is the best, i find five guys very weak in taste and execution. it is cool to have all those toppings as choices but, more often simpler is better especially if your quality is good.