Generally speaking, you want to look for an eggplant that is firm, smooth, and doesn't have any brown spots or blemishes. Smaller eggplants tend to be sweeter and have less bitterness than larger eggplant. Normally I recommend choosing small to medium sized eggplants for this reason.
Additionally, you can look at the eggplant stem to test freshness. It should be green and shouldn't have any mushiness or signs of mold.