This is a list of most popular or subjectively best price/performance thermal paste compounds (or TIMs = Thermal Interface Materials) compiled for my personal quick reference and comparison. I thought it might be useful for others, as well. It is not considered as complete at this stage.
Brand / Model Thermal Price  Density  Electrically Ease of Conductivity  Conductive  Use  ------------------------------------------------------------------------------------------------------- Thermal Grizzly Carbonaut Pad  62.5 W/mK $16.0 - Yes Medium Innovation Cooling Graphite Pad  35.0 W/mK $10.0 - Yes Medium Thermal Grizzly Conductonaut 73.0 W/mK $15.0 6.24 g/cm³ Yes  Hard Thermal Grizzly Kryonaut 12.5 W/mK $7.0 3.70 g/cm³ No Easy Thermal Grizzly Hydronaut 11.8 W/mK $9.6 2.60 g/cm³ No Medium Thermal Grizzly Aeronaut 8.5 W/mK $8.9 2.60 g/cm³ No Easy Phobya NanoGrease Extreme 16.0 W/mK $4.3 2.50 g/cm³ No Hard Phobya HeGrease Extreme 8.5 W/mK $2.9 3.73 g/cm³ No Medium Phobya HeGrease Standard 6.0 W/mK $1.7 2.50 g/cm³ No Easy Thermalright Silver King 79.0 W/mK $13.0 6.77 g/cm³ Yes  Hard Thermalright TFX 14.3 W/mK $6.5 2.60 g/cm³ No Hard Thermalright TF8 13.8 W/mK $5.0 2.90 g/cm³ No Easy Thermalright TF6 12.5 W/mK $3.5 3.60 g/cm³ No Thermalright TF4 9.5 W/mK $4.0 2.70 g/cm³ No Easy OurGD GD007 6.8 W/mK $2.7 2.50 g/cm³ No OurGD GD900-1 Silver 6.0 W/mK $1.9 2.50 g/cm³ No OurGD GD900 4.8 W/mK $0.25 2.30 g/cm³ No Easy Noctua NT-H2 - $7.1 2.81 g/cm³ No Medium Noctua NT-H1 - $4.8 2.49 g/cm³ No Medium Arctic MX-4 8.5 W/mK $4.5 2.49 g/cm³ No Medium Arctic MX-2 5.6 W/mK $1.0 3.96 g/cm³ No Easy Arctic Silver 5 8.89 W/mK  $3.8 - No  Medium Gelid GC-Extreme 8.5 W/mK $4.0 3.73 g/cm³ No Medium Gelid GC-Pro 7.0 W/mK $2.2 2.80 g/cm³ No Medium Gelid GC-Supreme 4.5 W/mK $1.1 2.55 g/cm³ No Medium Gelid GC-2 - $1.1 2.80 g/cm³ No Medium Corsair TM30 3.8 W/mK $2.3 2.50 g/cm³ No KingPin Cooling KPx - - - - Hard Cooler Master MasterGel Maker 11.0 W/mK $7.5 2.6 g/cm³ No Medium Cooler Master MasterGel Pro 8.0 W/mK $3.5 2.6 g/cm³ No Medium Cooler Master Extreme Fusion X1 9.5 W/mK - 2.6 g/cm³ No Medium Cooler Master IC Essential E1 4.5 W/mK - 2.5 g/cm³ No Easy Cooler Master IC Essential E2 3.5 W/mK - 2.4 g/cm³ No Easy Innovation Cooling Diamond 7 Carat 4.5 W/mK $6.0 - No Hard LT-100 Liquid Metal Grease 128.0 W/mK $7.0 - Yes  Hard
Data is provided as is – any corrections may be submitted in comments section below. While I try to be 100% accurate, in some cases data was not provided directly by original manufacturer, so they were obtained elsewhere from indirect sources (e.g. online shops product specifications).
 Thermal Conductivity roughly translates to better performance overall. However, the relationship between temperature reduction and thermal conductivity is not exactly linear, and it may differ for different cases (CPU or GPU, air/water/passive cooling, low pressure (push-pins) or high pressure (back-plate + screws) HSF mounting systems). Additionally, in good cooling systems, large heatsinks or air flow usually compensates for TIM layer variance and results are only within couple of degrees difference (0.5 ~ 3.0 ° C) between low/mid and high-end solutions. Higher viscosity compounds and pads tend to fill-in large gaps more effectively, but may “fail” with microscopic ones in low-pressure mounting systems, effectively resulting in larger surface contact resistance and lower performance, despite material higher thermal conductivity itself.
 Pad: Price for approx. 30×30 mm size. Paste: Price for 1 gram in USD at the time of publishing. Approx price was derived from popular online shops including shipping. Price varies between shops, shipping location (delivery country/address) and amount (package size). Smallest available package was chosen to calculate equivalent price for 1 gram. Bigger (bulk) quantities/packages are more economical, but not attractive to most ordinary users and hobby or personal use.
 Density should not be confused with Viscosity (see  below). Denser compounds appear heavier per unit of volume.
 While most compounds claim to be electrically non-conductive and non-capacitive, some may act like parasitic capacitance, and you should pay attention and try to be careful not to spill it around sensitive (exposed) PCB paths and components.
 Easy of use is relative quality directly related to viscosity (in case of pastes) or ease of application (in case of pads). For pastes, higher viscosity compounds appear thicker and harder to spread (also see  above). Higher viscosity = harder to spread or clean/remove paste later. For pads, some users reported how slippery pads are and that tiniest misalignment while installing HSF back can cause high temperatures.
 Pads come pre-assembled for many standard CPU/GPU chips. While thermal conductivity is higher and very close to liquid metal solutions, it’s thickness (Thermal Grizzly pad is 0.2 mm thick, Innovation Cooling pad is 0.125 mm thick) is very high and lowers the heat transmission compared to standard thermal compounds. As technology matures and allows for thinner pads, they may eventually outperform any conventional paste. Thermal pads cannot compensate well for surface height variation like the pastes do. Thermal pads are performing better in high-pressure HSF mount systems. This means that push-pins low-pressure mounting systems are in general not good candidates for pad applications.
 Liquid metal compound. Not suitable for < 10° C temperatures and sub-zero cryo cooling. Corrosive compound. Not suitable for aluminum heatsinks. May leave residue after removal.
 Liquid metal compound. Not suitable for < -3° C temperatures and extreme cryo cooling. Corrosive compound. Not suitable for aluminum heatsinks. May leave residue after removal.
 Liquid metal compound. Not suitable for < -30° C temperatures and extreme cryo cooling. Corrosive compound. Not suitable for aluminum heatsinks. May leave residue after removal.
 Data extracted and converted from online store. Not provided by original manufacturer.
350000 W/m²K x 0.0000254 m = 8.89 W/mK
 Not a conductor, but not a true insulator, either.
Which Thermal Paste Should I Buy/Use?
This question seems to be simple enough – depends on how deep your pockets are and how much money are you willing to spend. On the other hand, the answer is actually far more practical than that, if you understand couple of things about cooling performance and how it actually works.
First and foremost, there is no point of using high-end thermal compound solution with generic, lousy and loud stock cooler, hoping that that setup will outperform some other system using better cooler with larger fan (or no fan – passive heat-sink) with generic or cheaper thermal paste!
In my personal opinion, e.g. if you have a stock CPU cooler (GPU cooling solutions are nowadays usually tightly integrated with overall graphics card design), and you wish to improve your cooling performance (lower temperatures, reduce fan noise), the most logical step is to invest $15 in better cooler and average/mid-range/economical, instead of buying a high-end paste! If you are not too finicky about secondhand (used) coolers, you can get a really great one with larger effective surface area, many aluminum fins, massive copper base and heat pipes, accompanied with lower RPM fan (lower RPM = lower noise). Or it may be a completely passive cooling solution. In any case, that is a better move than investing in an expensive thermal paste alone.
On the other hand, if you already have a great cooler and you are satisfied with it, or you are simply stuck with what you already have, but you still wish to improve efficiency of your system (reduce temperatures, fan noise and electricity bill), then by all means – go for high-end.
If you want the best possible performance, you should go with liquid metal* or modern graphite or carbon nano-tube pads** solutions. Both are electrically conductive. In case of liquid metal paste, you have to be delicate not to spill it around on other components or surrounding PCB. In case of a carbon pad, application is very simple, just make sure you pick up the right size.
* Liquid metal solutions are not suitable for beginners. They require special application procedure and removal at later stage usually leaves traces or damaged IHS surface. You must understand the risks involved before use.
** Pads may outperform low- and mid-range grease solutions, but individual experience may vary and in some cases high-end thermal grease compounds may outperform pads by a noticeable margin. High-pressure HSF mounting systems are recommended.