Build 1: Early Damage -> this is used against casters in mid that don't have much poke but are wanting to get in your face and nuke you down. Its very rarely used and I suggest it only in extreme circumstances against champions such as le'blanc or annie. The bonus health from haunting guise and it's magic pen will allow you to win the 1v1 encounters and get kills. Even against champs like le'blanc. You max your W before Q in this build for the maximum damage.

Build 2: Early Survivability & Late game Power House -> this is used against safe mids that you can't really kill but are not too aggressive. Morgana for example, or alistar. In these situations you tend to get a lot of farm meaning you can go for the expensive late-game build but you do have lack-luster damage in the earliest team fights.

Build 3: Sustain & utility -> This is used against very aggressive mids like brand, cassiopeia or kassadin that you can't hope to beat under normal circumstances. Basically they are a better mid than you, so you need to focus on farm and play safe. You can still win the lane by pure CS. This will give you a strong supportive presence in team fights although your damage will be moderate unfortunately.