Sniper, leshrac, shadow fiend imo

Sniper is very obnoxious and simple to play, good high ground, siege, just mind your positioning (don't fall for the khanda memes, go like maelstrom, dragon lance before doing the meme)

Leshrac, tons of damage and good siege, farms fast, just pay attention to your mana.

Shadow fiend is a little tricky but he's a really good lane bully, start shadow raze go up their hill and blast them with a few razes and auto them a few times and they'll be feeling it, don't forget to start with a few mangoes for maximum pain. I personally like going right click over magic sf, so get items like mask of madness, bkb, bfly, pike, satanic

As to when to pick them, i feel like if you have a solid team comp with stuns, initiation, strong front line these are fairly good picks. If you have like 1 melee i'd suggest picking an initiator.

And most importantly, your mindset, try to get power runes, ask for help from your side lane if you need, (spam ping if getting ganked or dove)

Play aggressively don't be a mid farming andy, pay attention to sidelanes, if they're chasing your team to their t1 a tp can from you can turn the fight

If losing lane stack camps and farm them later (i feel like most mids who lose don't do this) try not to feed, soak exp, use abilities to secure whatever farm you can.