Terms: P* (Polarstar), OOTB (out of the box), DI (Drop In), CR (Cylinder Replacement), MC (Mancraft), W (Wolverine), ME (Mechanical), FE (Fusion Engine)
So Im in the US, which means HPA is far more common and available. As far as I know I have used quite literally every HPA system out there, barring a few unique or uninteresting ones to me (Russian "bear", Im looking at you). Daytona, Escort (pre-Daytona), W Inferno, Reaper, Reaper M, SMP, Hydra, Bolt, Bolt M, P* FE v2, FE v3, FE 249, Jack, F1, F2, Valken v12, Tapp HPA adapters, CQBRussian HPA taps, custom one-off HPAs, Redline engines, MC sDiKs, and probably a few others Ive forgotten. Ones I havent used include the P* Kythera, Redline MilSim, the aforementioned Russian "bear" (aka MedVed), and as stated single-gun engines like the old JACs.
P* is, by far, my favorite for a few reasons. First, LCD FCUs. Wolverine engines are a PITA to adjust to exactly what you want - the older versions using two screwdriver dials, and the newer having an extremely basic FCU with lights to show settings. P* is all written out on an LCD screen, like a calculator screen.
Second, are FEs. The only true drop in and go engines (besides the v12, but we dont talk about the v12) means you can keep your AEG gearbox together for a rainy day, to shove into another gun, or to use as spare parts.
The F2 has the power of the FE but is a CR engine, like almost everything else on this list bar the Daytonas. Basically this means it uses your old gearbox.
Closely following this is the Wolverine Reaper. While Ive found the F2 to be an easier drop in, theres no denying how quiet and air-efficient you can make this top of the line wolverine engine. If the FCU was better OOTB it would be neck and neck with the F2, and if Id not had to work so hard to get Reapers to work well in some odd guns itd be ahead of the F2.
W then makes an interesting engine - the Hydra. The Hydra is made for all those offset nozzle guns. Im not a huge fan of the engine itself, having not-great experiences with it in my P90, but if you have an offset gun and dont want to design an entirely new gearbox (something I have done a time or two) its your best option. P* has made offset nozzles for the F2, but they arent too common (still looking for a PDR-C nozzle...).
A few notes about some of the other engines. The V12 is trash. Easily the worst HPA system out there and not worth looking at. The Jack is the worst engine that is actually worth the time to install in your gun. Personally Id avoid it if you can. The only one I still own is in my Marui MP7, as thats literally the only engine that will fit in it. The F1 is just a closed-bolt Jack. Daytonas are extremely fun, but some of the most time-intensive, expensive, inaccurate, PITA to work on systems out there. When they work its cleanup on aisle everyone-on-the-field's pants, but they are just such a hassle. I dont put much stock in the ME systems like the Kythera, Reaper M, or Redline MilSim as I dont run many DMRs - though the option to forgo a battery entirely is awesome if you will never go auto. If I didnt always run LMG or SMG at MilSims Id probably have a few of these engines installed over the many FEs, F2s, and various W engines I do have.
Now, since you didnt mention what gun I will go ahead and talk about the bolties. You have two main options - W and MC. W has the two Bolts, while MC has the sDiKs. Bolt is the older model that uses a battery, Bolt M is ME and doesnt use a battery. I have a Bolt in my VSR and its great. A real ninja converter of an engine, I can be audibly invisible 20 feet from a person with a good foam suppressor. The sDiKs are sorta like the Hydra to me - they are available for tons more oddball bolties, but I could never get the performance out of them, their installs are odd and have weird placement (like my MSR wanting the hose to go out the trigger?!), and they just seem clunkier than the Bolts.
All in all I probably currently own some 25+ HPA guns of various makes and manufactures. Some notable ones are my Inferno PKM, FE 249, Daytona MG36, L85, and M27, Bolt VSR, Jack MP7, FE PP-91 (see the "Mosquito' thread on here), F2 L86, F2 AUG HBAR, and F2 G28LE/417A2. I am happy to say more about certain engines if you wish. You will also - regardless of what engine you get - need a good tank and regulator to run said gun. HPA is expensive to get into, but IMO needs far less effort to tune, and breaks far less than AEGs. That being said I keep a steady supply of HPA, AEG, and even spring guns in my collection, so I can use whatever happens to strike my fancy on a particular game day.