I don't know if there's a single thing I enjoy the most.

The fight scenes are pretty exciting, though they're not necessarily the highlight. I will say that I think Rich does fight scenes better than a lot of authors because he has a strong sense of keeping the narrative elements in play. Like how Elan's continued defiance of his father (and said father's attempts to assert authority) keep going throughout the fight scene. Or how the Crystal Golem fight climaxes not with Haley pulling off some clever combat maneuver, but by some brutal honesty helping to resolve a long-standing character flaw.

Even within more self-contained fights (eg V vs. Laurin), he's still pretty good about making sure there's an emotional layer (Laurin's apparent resentment of Elves, which is doubles as a nice bit of world-building) as well as spicing it up with good visuals.

I think, also, he knows when to end a fight. V vs. Laurin it lasts two strips. Even ones that go on longer have a good reason. Durkon is too determined to see a project through and Minrah too belligerent to retreat when they should've and now they're facing the consequences.

Now, all that said, the battles aren't my favorite parts per se. Some of my favorite things in no particular order:

-Gags and one-liners. Rich had a knack for giving his characters memorable moments without them feeling forced. "Big faces are getting big maces!" or "No I don't [have complicated feelings about Durkon]! And maybe if I stab something enough, they'll go away!" or even the "Nale nail Not-Nale" monologue are just so good. And there are plenty of similarly great visuals.
-Characters doing impressive things. Haley forcing Tarquin to let go of the ship's hull by shooting at his eyes and forcing him to use both hands for arrow catching (all with a broken arm). Vaarsuvius coming in at just the right moment to bail out the Order from Tarquin's army. Almost everything O-Chul has ever done.
-The surprising amount of pathos that can be brought forth by silly stick figures. Durkon's death or Roy finally returning from the grave.
-The plot and world building. It's good stuff and I'm eager to learn new things about both.
-A wide array of characters and motivations. A lot of works fall into a sort of dominant thing. Whether you have by-definition righteous heroes and vile villains or you have "everyone's cynically working towards their own ends" or it's all idealistic motives with questionable methods, a lot of works settle in a world-view and most characters conform to it, with a few outliers who exist to be proven wrong. OOTS feels so vibrant and alive in part because an ends-justify-the-means villain like Redcloak can exist beside a Evil-for-the-Fun-of-It villain like Xykon and neither feels out of place, and neither do other villains who range from well-intentioned extremists (Miko) to I Know My Place, You Learn Yours types like Tarquin.

That's just a few things why OOTS is perhaps the thing I've been a fan of the longest and most consistently in my 38 years of living.