I use YYE bulk string. I can’t afford the fancy kinds. 
I do plan on trying out Kitty String
Markmont’s Dragon string from Toxic is my absolute favorite right now. Adore it. It’s fabulous.
Homemade
BG1 Yellow Jackets, Choas, and Kitty String
Epic string
Mostly plain old slick 8, some slick6, and cotton on some loopers.
tried toxic its okay. lasts long but it whips a little too fast and is a little to thick so i normally just use yye bulk string.
Used to use HamString, but now I use YoYoStringLab Type X
YYE. Or Dragon string. 
YYE bulk
MB strings exclusively.
kitty or twisted e string
G-String, NOLY Format - Good for performance, and durability. It’s pretty rough on the hands though since it incorporates Nylon. My go-to if I want to bust out some sick moves. It’s too expensive IMO to use on a regular basis though.
YYF Bulk Poly - Comes in the perfect length for me, moderate comfort while playing, whips/slacks are pretty good, but not amazing since it’s not a exceedingly thick or stiff string. Holds tension for awhile, but does need to occasionally be adjusted. Biggest pet peeve is that they don’t have a very long life and they tend to snap on you if you don’t pay attention. Overall good though.
Kitty String - Everything that bulk poly offers, with a longer life, plus it’s like playing with silk after it’s broken in. Seriously, this stuff glides across your fingers after a little bit of playtime. Also the slacks are -slightly- better in my experience. I use this when I decide not to be lazy, seeing as you have to tie your own loops and adjust the string and all that. - My main string
Toxic String - I’ve used these before, wasn’t a fan but that might just be because of the format I got. The Rainbow version didn’t look very appealing in person and the string was extremely thick. I do know people who loves these though… Different strokes for different folks.
YYE Bulk Poly - I’m a tall person and in my experience these are tied a little short for my tastes. Performance wise it’s what you’d expect. They get’r done for the most part, and are pretty stellar once you first sling em on your yoyo. However, they dwindle off quickly and lose tension very easily in my experience.
I’ve had the opportunity lately to try out a lot of different strings, and in my opinion there’s no ultimatemazingohmygodthisistheshizz string. Everything I’ve played has strengths and weaknesses, and some might suit you better than others.
String length - This is the major factor in whether or not you’re going to like a string. I see a lot of people throwing their yoyo’s with their throw hand up to their head because their string is extremely long. You have to cut your string according to what’s comfortable to you and what play style you’re going for. If you’re a speed player, incorporating fast movement without too much tech you’re going to want a shorter string. If you’re a technical player you’re going to what a medium length string so you’ll have room to manuever, hop, wrap, and slide through your tricks. If you’re more flowy you’ll probably want a -slightly- longer string to allow for your motions to be consistent, and flow into the other.
String Thickness - The thicker the string you buy, the easier it’s going to be to do slacks and whips because the string has more weight to it. However, because the string is thicker, it’s not going to work as well in smaller gapped yoyos, and in tricks that incorporate a lot of layers being fed into the gap. Similarly thinner string make it easier to feed large amount of strings into your gap, however you lose some of the whip/slack capability that thicker strings have. Normal strings provide a happy medium between the two. It really depends on what you want to do.
Material - Most people use polyester, however there are some niche materials used (kevlar/nylon). However, there’s several different types of polyester that will give the strings a different feeling. Part of the reason why bulk strings are cheaper than premium strings is because they use a less than optimum fabric. Using different types of polyester gives the string different qualities IE when people describe string as being “Soft, Stiff, Bouncy” it’s more than likely because a different type of polyester was used in the process.
I’m going to stop there – Why did I write such a long post? O_o.
I’ve just been thinking about string a lot lately.
All of this is from my own experience.
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I throw Hamstring all day bro.
Kitty String. It’s affordable, comfortable, and uniform like bulk string should be, but lasts a little longer and feels a little better than YYE string.
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I have a small pack of Toxic I plan to try in the near future.
Highlight, yye bulk, dayglows, whatever u call em nowdays.
I’ve been throwing a lot of bulk string, like Kitty string or like Expert string, since it is hard enough for me to keep up with all the orders, so I don’t usually go and use my own string
plain old white cotton. i have two cones of old type 10 from which i twist regularly, along with several hundred type 8 i pull from for a different feel.
i find the idea that you need “this or that boutique string” to do what you want to be as fallacious as needing “this or that boutique yo-yo”.