AAC is generally more modern and better for lower bitrates, but AC3 (also known as Dolby Digital) has the advantage of being able to be transmitted in 5.1 over SPIDF optical connections, so it can allow for surround sound in older setups that may not otherwise be able to recieve digital surround sound.
Opus is slightly better than AAC at matched bitrates, slightly less commonly supported, and totally open-source. It’s a fine choice as well.
Also of note because of its use for anime encodes is FLAC, which is lossless and therefore results in much larger files, but will always have the exact same quality as the original audio it encoded, so it’s excellent for archival quality.