You will get 100 different opinions from 100 different people as to which is "the best" and what is junk. The best thing to do before dabbling into mechs is to educate yourself on them. Familiarize yourself with all of the safety issues that go along with using mechs. Then you need to figure out what you want. Are you looking to chase clouds??? Are you looking for flavor over massive vapor??? You want stealth (18350) or a baseball bat (26650)??? After you figure that out you can narrow down some of the choices.
A good example for you. The other day at work i 'confiscated' a couple batteries from a customer before he hurt himself. This guy had only bothered to watch the "look at my huge cloud" videos and didnt pay any attention to build safety. He was running a .1 build on 15A batteries. Damaged batteries at that. He neglected to adjust the pos pin to make room for a button top and smashed the button torqueing the switch on the mod. The ring around the top was discolored on both batteries from where they had partially vented. Using 15A batteries on a build thats going to pull 40A+ can only end badly.
Before you start building learn ohms law(if you dont already)...learn it,live it,love it. Safety is paramount for yourself and the people around you.
I recently moved up to a 26650 mod just for that extra amperage the bigger batteries carry. So far its working out nicely. Once you get used to the size of the mod 18650 mods seem tiny in comparrison,almost eGo like.
And the batteries arent that much more expensive, only a buck or two more.(at my shop at least)