TL;DR: Space is big, you wouldn't really have too try to hard - It could take the aliens at least 27 years just to find you, or as much as as 1.75E19 years, but if you must, try hiding on something fast.
It really depends on how bigger an object you want to hide. Remember, it's called 'space' for a reason.
Anyhow, you want to hide from genocidal alien berserkers, so let's assume it's a whole planet for the sake of the argument. That's perhaps a 30,000Km radius - The earth is about 6,000Km radius, so that's a conservative number for a terrestrial planet. As you suggest you need a decent energy supply, I'll allow for that to.
The ultimate factor in where you hide will the the sensitivity of your enemy's sensors - IF they use some form of radar or electromagnetic phenomena that by definition propagates at the speed of light, you don't even have to hide. - The Milkey Way is 100,000Ly diameter, so 'hiding' (in plain sight) on the opposite of the galaxy gives you 100,000 years to build super weapons before they even know you're there. However you've specified FTL speeds of 20 Ly / Day are available, so let's assume they dispatch thousands (trillions) of remote probes that must find you then fly back to report. That gives you 10,000 days (27 years) before the probe can find you and report back to the far side of the galaxy. Therefore: Scenario A: The other side of the galaxy, where ever is most convenient in terms of resources and energy.
Again, if you sensors have a (relatively) low resolution or sensitivity, anywhere with sufficient background will do. If we use a real science base, it just has to be the right background - no information can propagate faster than light (even gravity waves), as we currently understand physics, if your enemy uses electromagnetic sensors you hide near (or in) a bright star (Say a blue supergiant - which has a luminosity [brightness, basically] of 1,900,000 times that of our sun.) The heat would be a problem at 10-50 thousand kelvin, but at least you'd have your energy sorted.
On the surface of most planets, it would depend on how far away your enemy was - I can't give you an exact number because it would depend on the sensitivity of the sensors to electromagnetic flux, but consider that many chemicals produce an EM spectrum very similar to that of industrial activity and metabolic processes (IE the presence of elevated quantities Nitrogen, Oxygen, Carbon, Phosphorous, and Sulfur) exist on planets that could hide a smaller colony unless a ship got quite close relative to the resolution of their sensors - Consider that our best pictures of Europa - a moon of Saturn, have a resolution of about 1,000 Km^2 per pixel - easily enough for a small colony to go undetected. Carbon Planets (https://en.wikipedia.org/wiki/Carbon_planet) would be a good bet assuming you didn't vent phenomenal quantity of Oxygen into the atmosphere (That means in excess of 1E10 Kg, where it would start to be visible on a current technology spectroscope from about 20 Ly). You mentioned terrestrial planets with a large amount of volcanic activity.. I wouldn't bet on it, as they would (terrestrial) planets would be a priority in my xenophobic hunt if I was running it, because they tend to be ideal for life to evolve - remember their are 100,000,000,000 planets in the milky way, so searching each on a closest first basis could still take 36,986,301 years (assuming 10 planets per solar system, 27Ly average between each system, 20 Ly per day travel with instant acceleration and deceleration). So by extension of that logic, B: Any planet you wouldn't expect to find life, perhaps the atmosphere of a gas giant - It might be hard to get materials there.
Galactic center... Good option at first: High flux makes it hard to identify individual targets, lots of energy and mass for industry.... Except - when you hostiles start to get close, they can get more fidelity from given sensors - Imagine physically moving closer to an object that can't be photographed in any meaningful way, because it's 10,000 Km away and you're using your cellphone camera. Thus you start to get seen again.
[Incorrect relativity deleted]
My favorite place to hide would simply be on a large moving space dock, making no particular effort to hide. As I've already pointed out, a procedural search of the galaxy could take an unreasonably long time, but the could also get lucky in the first planet they go to. However, if you make yourself a moving target, you can lower the numbers even further. Using the same numbers as before, if they can check (on average) five planets a day, your change of being found on any given day is 5E-11. To make this easier, let's say they enemy can check any planet within 20Ly of their location instantly. This increases your chance of being found to about 1.25E-11. If you then cover 20Ly in a random direction per day, your chance of being randomly located is 1.25E-11 * 1.25E-11 (Your chance of being on a given 'sector' on the day, times theirs.) per day. 1.56E-22 per day change of being found give you a predicted stealth time of 6.4E21 days = 1.75E19 years. Of course, their is a low chance of you randomly encountering them a lot earlier, or alot later, but with that kind of margin, I'd be willing to bet my life on lasting at least the 10,000 years you want.