Mr. Karl O Marx,
Privet!
IMO, PADI is your best option as it recognized world wide and the standards set in place are very good. Of course as a PADI Divemaster I am a bit biased. In reality there are several choices, but clearly this would be the easiest and most recognized choice that will allow you a clear path to continue advancement in diver education. You will start getting a few businesses directly trying to get you to sign up with them. As an independent DM who runs a scuba club part time I would suggest looking for a place to get certified on the Andaman side in February. The season for the Gulf is late May to October. I would avoid Pattaya for that reason until the season changes. Conditions in the Phuket/Krabi area would be excellent.
Think budget, time of stay and what else you may want to do besides diving. I really liked Phi Phi Lei. The Holiday Inn resort is on a quiet end of the island.
Scuba Diving-Holiday Inn Resort Phi Phi Island, Krabi, Thailand You can get certified right at the hotel and do some great beach dives and boat diving within 15 minutes journey. The conditions will be ideal that time of year. The down side is the price. You can find cheaper places, but this is a nice place to stay.
Getting there is easy. Fly from BKK to Phuket and arrange transfer through the hotel. A ferry will take you there from Phuket.
I would consider staying at least 6 days. E-learning may also be a consideration. If your English is good, check out:
PADI eLearning Registration
I am not sure if the dive shop at the hotel uses E-Learning yet but email the hotel if your interested. I think their dive shop may have a link off their home page or just about to add one. Most places are starting to offer this route as it is great for people traveling. With over 80,000 people getting certified in Thailand each year this is real consideration for the tourism market.
This will allow you to complete theory before starting your holiday and maximize your time. Its still relatively new, but you may want to look for an operation that offers this. Going the manual route is good as well, but this will make things more enjoyable IMO.
Cost: E-Learning 120USD, cost for the rest varies. Everyone is usually very similar so don't think shopping around price will be a big issue. I would let weather conditions, budget for hotel and closeness to the dive sites be your guide.
Filling in your profile would also be helpful as to let the folks on SB better know where you are coming from. Advice is plentiful in these parts.
Good luck,
Arizona
