Some of you might get all nitpicky about this, insisting what is good with steak may not be as good with chicken or beef or whatever. So be it. List the side dish you most like, specify what you will, justify it if you so please.
Removing joke that will get me modded…
Baked potato.
My mom’s traditional Newfoundland savoury stuffing. No debate. It’s fundamentally a very simple dish, but is just ooooh soooo gooood. It’s probably too late for you all to order the proper Mt. Scio savoury this year, but if you plan for next year, you’ll thank me.
Twice-baked potato.
Scalloped potatoes!
French bread with unsalted butter
Corn on the cob
Mixed vegetables roasted in olive oil with fresh rosemary
Mac & cheese; duh.
There are some sides that I make that I sometimes just have on their own.
A supposedly German potato salad with bacon and eggs in it.
Fried potatoes with pepperoni, onions and tomato.
Charred Brussel sprouts with almonds and chili
My most universal side is green beans (I have a couple of versions).
Most OMG so good side dish would be my baked beans made with sausage and peppers.
Well, if we’re going through potato dishes - pommes dauphinoise. (Or perhaps I should say potatoes au gratin, as the wiki says that’s the American English)
If you can’t be bothered with that, there’s a lot to be said for new potatoes roasted with rosemary.
j
Potatoes cooked in the French style, aka french fries. My gf sometimes criticizes my love of fries.
Mashed potatoes with lots of salt & butter and then topped with corn.
Baked potatoes rolled in olive oil and salt (before baking).
This is an inherently paradoxical question, because any sufficiently good side dish will become a main dish.
When cooking a serious meal, I try to make every side as good as the main dish. But the main dish differs due to its billing, portion size, attention, possible expense and quality, traditional role and when it is served. Not just its goodness. You’ve also had plenty of meals where the sides outshone the main.
Rice, be it pilaf, spanish, dirty, whatever. I like baked potatoes but when I have a steak in a restaurant I’m a little bummed if there’s no rice on the menu.
And of course, nothing beats french fries (though oddly enough, I don’t like them with steak).
I know a place to get this fantastic Red Potato salad with Bleu Cheese and Bacon. Man, it’s GOOD!
Depends on the type of food. For Mexican food, I love refried beans with basically everything. It would be my go-to side dish. Black beans are also high on my list…in fact, beans in one form or another would be my preference.
For American-type food, I’d go with potato in one form or another (fries, baked potato, etc). Baked beans are also a favorite of mine depending on what the main course is. Mac and cheese is also a favorite of mine, even though I have a bit of an issue with cheese…still love the stuff though.
Yeah, I went there too. 
French fries, properly done. Goes with anything.
Hot, fresh potato kugel. Absolutely melts in your mouth.