String choice is one of those topics that gets more complicated than it needs to be. Here is what I actually use and why, plus a few things worth knowing before you buy.
Gauge matters more than brand
String gauge is the thickness of the strings, measured in thousandths of an inch. The first number in a set (9, 10, 11) refers to the thinnest string. Lighter gauges (9s) are easier to bend and gentler on your fingers. Heavier gauges have more volume, more sustain, and hold tune better. Start with 10s if you are not sure. Most guitars leave the factory set up for 10s.
What I use
My go-to is D’Addario. For E standard and drop D tuning I use the Nickel Wound XL 10-46, and that is what I would recommend as a starting point for most players. They are consistent, widely available, and have a good bright tone when fresh. Ernie Balls are a close second and also excellent.
On coated strings
I am not a fan personally. Coated strings feel different under the fingers and I find the tone a bit muted compared to uncoated. I know plenty of people swear by Elixirs for how long they last, and if coated strings work for you then fine. For me, I would rather change uncoated strings more often and have them feel and sound right. Try them if you are curious, but do not be surprised if you prefer uncoated.
Watch out for fakes
If you are buying from Amazon or eBay, be careful. Fake D’Addario and Ernie Ball strings are genuinely common. They look identical to the real thing until you try to tune up, and then you find out the hard way. Cheap knock-off strings do not hold tune, break more easily and generally feel horrible. Buy from a reputable source: Gear4Music, Thomann, PMT or your local guitar shop. Avoid random Amazon marketplace sellers with no reviews and suspiciously low prices.
Do not change gauge without a setup
If you switch string gauge, the guitar needs a setup. Different tension means the neck, action and intonation all shift. A gauge change that should feel better will feel wrong if the guitar has not been set up for it. This catches a lot of people out.
Change them more often than you think
Old strings go dead. They lose brightness, feel sticky and go slightly out of tune more easily. If you play regularly, every couple of months is about right. If you are gigging, more often than that.
Re-stringing
If you have never had strings put on by someone who knows what they are doing, it is worth trying once. Proper winding technique makes tuning more stable and shortens the break-in period. I offer re-stringing with a full clean from £20 plus strings. Details here.