Tennis strings are just as important as the racket because you can get a racket to demo and find it's just awful, but if it would have had a different string, it might actually have been a really good fit for you. You need to expect to experiment with strings before finding what works best for you, your racket, your style of play, and perhaps any injuries that you may be dealing with.
Gut strings are sometimes called "catgut" but actually no cats are harmed making gut strings but cows aren't so lucky. Gut is the most expensive choice and perhaps not the best choice for most players. Gut tends to fray and if you’re hard hitting using a lot of topspin, you'll be restringing your rackets frequently. For playability, you will get a better feel from gut as compared to multifilament (and certainly compared to polyester), but I doubt most recreational and even moderately competitive players would notice a real improvement in their game with gut. Personally, I've played with many brands and types of gut and occasionally I've had a magic string job that I hoped would never break. Sadly, it's only been occasionally, so you'll see that reflected in the best list that follows.
Multi-filament strings are most commonly made of nylon or other materials and are intended to play like gut, or at least as close at the manufacturer wants to get for the price. There are some multi-filament strings that really do play like gut for a time, but that time is usually just before they break. Still, these strings are very cost effective and offer great control with less strain on your arm. All other things being equal, I'd suggest beginning players start with these strings until they develop a game that can really benefit from gut or polyester.
The polyester string, or "poly," is the other main alternative. It's much stiffer than multifilament strings and variations of the poly design are available from firms such as Luxilon and Babolat that have proprietary blends of poly and other materials. Sometimes you'll see a polyester string described as a monofilament so don't confuse that with multi-filament softer strings. Ask a tennis pro if you have any doubt about what you're getting.
The options for strings vary considerably. There's a range of tension from loose to tight, and the same string will play very differently at the limits of the recommended range. There's the string gauge, ranging generally from 15-18, with 18 being thinnest. 16 is the common gauge that most players will find but again the same string in different gauges will play very differently. Finally, you can opt to hybridize two different types of string. For example, Roger Federer famously uses Wilson gut in the main strings at 48.5 pounds of tension and poly (Luxilon Big Banger Alu Power Rough) in the cross strings at 45 pounds of tension.
The strings on this best list have some common characteristics. Of the strings I've tried, these are the best in their class. These strings play consistently from one string set to the next and they generally don't play too much differently after several sets when their tension is much lower than the original string job. Still, string tension, gauge, composition, and feel can be very subjective and will vary by your choice of racket. So ultimately you should use this list as a starting point for your exploration of what works best for you.
VS gut is the gold standard for gut strings. It offers great "ball pocketing," where you have the sense that the ball is on the strings longer, and it holds string tension better than polyester and multifilament strings. Read Full Review
Of all the brands and variations of gut strings, Babolat's VS Touch remains the prime choice and when strung right, the feel of VS gut is so soft. The degree of power and control it offers is so addictive that you'll want more but the only drawback is the price. It's routinely over $40 per packet and that's before you pay a stringer to fix up your racket. Babalot advertises a new BT7 technology added to the string they state adds 15 percent to its durability, so that might keep you away from the stringer for one more session.
If you have a fast swing then you'll want to consider hybridizing the gut with a polyester such as Luxilon Big Banger Alu Power to get a compromise of playability and power. There are certainly other brands of gut strings to consider, such as Klip and Pacific. I've played with them as well and while cheaper than VS, they’re not quite as nice. Wilson gut, Roger Federer's choice, also deserves a try but it's more expensive.
A cost effective poly string that’s easier on your elbow than many others. It offers plenty of power on a fast swing, yet those hitting with accentuated topspin will find they have all the spin they need. Read Full Review
Solinco makes great strings and my favorite is the Tour Bite Soft. This is a poly (monofilament) that offers considerable pace on a fast swing and accentuated spin. If your ground stroke game relies on topspin and a hitting aggressively to conservative targets, Tour Bite Soft can complement your strengths.
The "soft" version is easier on your arm and as a former victim of tennis elbow, I've tried many poly strings but very few seem to meet with my elbow's approval, especially on slice shots. Tour Bite Soft is an exception and it's currently my string of choice. It plays well in my Head Graphene Radical MP at any tension from 48-54 and I don't find any need to go for a higher tension. I think this string is a "hidden gem" from Solinco and I recommend you try it too.
This is a classic multifilament string with good manufacturing consistency. It has a soft feel even in a stiff racket. The power from this string is superior as compared to many other multifilament strings. Read Full Review
Wilson NXT (and its variations) is a great multifilament string. At times it plays like natural gut, but that usually comes after breaking in a new string job. The string will start to fray after a while but you don't have to feel like you need to cut the strings and start over. They will play remarkably well and perhaps even more like gut during this stage.
Eventually, the last strand gives way and you're back to the stringer. Wilson rates this string as giving less control than power, but I think the 17 gauge version gives very good control. It offers less spin control than Solinco's Tour Bite but as a multifilament string that's okay. It's priced under $20 per packet, so it's about half the cost of natural gut.
A revolutionary string that allows a skilled player to impact tremendous power and spin. The string is stiff but responsive even at much lower tensions. Because it's so stiff it very rarely breaks making it a go-to string for many professional players on the tour. Read Full Review
This best list for strings couldn't be complete without including Luxilon Big Banger Alu Power. This string, or other newer related versions from Luxilon, are used by many professionals and has been credited with allowing the pros to hit incredibly fast shots simultaneously with incredible spin. We all want to perform like individuals who play tennis for a living and serious players (NTRP 4.5+) can find these strings to offer an improvement in their performance too.
There are other options that offer comparable features, for example, Babolat's RPM Blast. The drawback many players report is that because these strings are stiffer than normal strings, when you pull them out of the packet they feel like wire but the resulting effect on the arm is greater. I've seen some reviews of the Luxilon series suggesting troubles come not from the string but from poor form which may be true. On the other hand, those of us who aren't professionals likely have some limitations in our form.
I've tried many different Luxilon strings and the Alu Power Fluoro is probably the easiest on my arm, but not easier than Solinco Tour Bite Soft. If you’re a dedicated player looking for an extra edge in your game, try Luxilon strings and then decide for yourself if you can benefit from these strings the same way the professionals have.
NRG2 is a great multifilament string. It sacrifices a little in spin for more in power and you can find it for under $15 per packet so it's cost effective relative to other choices. Read Full Review
Technifibre makes a wide variety of strings, and I have to go with Technifibre's NRG2. It's a powerful string but I've found it to be much friendlier to my arm than Luxilon (even the Alu Power Fluoro) without sacrificing too much power. While power sounds great, ask yourself if more power really fits in with your game. The NRG2 string has an anti-abrasion coating that Technifibre calls "silicium pyrogene lubritec" which makes the strings last a bit longer than they otherwise would. One other nice thing about NRG2 is that you can string most rackets at low tension and still find the string plays very well with a softer feel.