What is the best treatment for otitis externa?

Ear drops. For uncomplicated otitis externa that's not severe, drops like cortisporin (neomycin polymyxin and hydrocortisone) otic work fine. Severe cases may require different ear drops, and maybe oral antibiotics if it's really bad. To prevent, use drops that are a mix of 2 parts vinegar & 1 part rubbing alcohol- let them run in & right back out any time you get water in ears; don't use if already infected.
Antibiotics +steroid. Topical antibiotic drops in combination with steroids will help with this situation in most cases. Occasionally a cotton plug called a wick needs to be inserted into the ear to hold the drops in place if the infection is severe.