Welcome!
I'd say buying a Megapack is the best choice because:
- You have the chance of getting 6*s
- You have the chance of getting 5*s
- You are almost guaranteed to get 4*s. If they are good, you can keep them for your team and if they're not, you can max them and then sell them for TE
- You usually get special items for events, pots, or even TEs in them.
I'd say keep your money until there's an event you like or one that gives easy prizes. Upcoming event (Arena) supposedly gives lots of TE