it very much depends on your requirements.
If money is not an object, the best of the best is a "Masterline" electric booster.. they are quiet and runs on electricity and is completely O2 clean. When I grow up, I'd love to have one of these.
Then when looking at pneumatic boosters, there are a few brands you can consider, with the most widely know being Haskell.
for the most part, you will want to look at air driven boosters in 3 configurations.
Single-Stage/Single-Action: (Model numbers start with AG) - These are the basic pneumatic boosters that uses low pressure drive air to boost source gas up to high pressure.. they use lots of drive gas and they are slow.. the output pressure depends on the ratio between the drive-gas piston and the output high-pressure piston.. The ratio is the number in the series.. i.e. AG15, AG30, AG75 , etc.. the lower the number, the lower the ratio and the lower the output pressure.. but the higher the output pressure, the higher the ratio between the drive-air and high pressure pistons, but that means slooow speeds.. for example an AG-30 is twice as fast as an AG75, but can boost to much less than half the pressure output pressure. Also, the bigger the difference in supply pressure to output pressure, the more drive gas pressure is required and the slower it boosts. depending on model, it can usually only effectively suck source gas down to about 400psi if you need a 3000psi output.. for Helium, thats a LOT of gas in the source tanks.. These are fine for rebreather diving, but they are a little slow for OC.
Single-Stage/Double-Action: This is the AGD (the D means Double Action) - this is essentially the EXACT the same as above, except it has 2 high pressure chambers, one on either side of the low pressure piston, but on each "action" or reciprocation of the low pressure piston, it boosts.. these are TWICE as fast as the above boosters, but also use twice the drive air volume. they are fast, but can only boost the same ratios as above and the same problem with waste source gas (350-400psi left in bottle..) They are fast and pump large volumes and good for OC, just gets a little slow when source tanks are low.. (I have one of these and they work just fine... )
Double-Stage/Double-Action: This is the AGT series (the T means Double sTage), These are GREAT dive boosters because they are essentially 2 single stage boosters connected in cascade and the one feeds into the other.. for example you can get a AGT 15/30 that can suck gas and compress it on the 15 side, then feeds it into the 30 side and compresses it more.. this means that you can have a HUGE difference in source gas and output gas pressures without losing too much effeciency.. disadvantage here is they are somewhat slow and uses lots of drive gas. but they are GREAT at scavanging source gas bottles.. you can easily suck a Helium tank down to 100psi with a 3000psi output.. These are best of both worlds and the best diving kind of booster you can find.
Now -ALL of the above boosters can be cleaned and serviced for Oxygen use.. BUT the service kits are expensive and you have to buy the kits per stage.. ie if you have an AGD or AGT, you need to buy 2 kits.. I paid about $600 for my O2 seal kit for my AGD30 last year..
Lets look at models.. and which are best for diving..
the 75 series have HUGE output and input pressures, like 20,000 psi.. but that means the HP piston is the size of my pinky and they are slow..
the 30 series is a good balance since they allow 9000psi input and output iirc. I have a AGD30 so its a double ended booster but single stage..
the 15 series is ok for low outputs and inputs and really large volumes.. 3000psi iirc..
now, you can start to combine these numbers when you look at AGT (Double Stage) boosters..
AGT 15/30 means it has a 15 on the one end and a 30 on the other, so source gas is no more than 3000psi and output is 9000psi max AND it cascades... this is the BEST booster you can buy for boosting Helium, Oxygen and Argon..
AGT 30/75 is good for boosting from higher pressure banks (up to 9000psi source) and can do 20,0000psi output...
So - in summary.. depending in your needs and budget.. the best booster for OC diving is AGT 15/30
But, all of them work, some are just more efficient than others..
Hope that explains it