This is a loaded question, but if you are set on running 275/35-18 square (which is a great way to go), we suggest either 18x10" ET25 or 18x10.5" ET22 wheel specs.
The 18x10" ET25 (or ET20) is the most popular wheel spec, and will provide most 275's with plenty of sidewall support. The rear fitment can leave something to be desired, so many add 10/12mm spacers (slimmer spacers if you are using ET20).
For enthusiasts running compounds that run wider than others, we prefer 18x10.5" ET22 for added sidewall support. The 18x10.5" ET22 fitment is also more flush in the rear, which eliminates the desire to run spacers. Assuming you have negative camber dialed in and suspension that does not compromise inner strut clearance up front, this is a fantastic setup that is easy to implement.
Here are some photo's of the 18x10.5" with 275/35-18 Dunlop Star Spec ZII (extreme summer) tires. Note r-comps and slicks will run a touch wider, and will benefit from the 10.5" wheel even more.
- Ryan