The "Classic" alchemist build is the most used one. It provides you a lot of regen from radiance and acid spray with octarine core and a lot of survavibility and stats with manta and AC (or s&y). The blink dagger is amazing cause you can pop the potion and blink stun at last second.
Starting items are normal. Quelling blade is a must for easier last hits and more gold. Stout shield for harass and some regeneration. Bottle and more tangoes for more regen and more lane sustain &trading. Treads and aquila provide a lot of amazing stats and much needed armor and attack speed. Combined with your minus armor, stun and ult you can be a very strong 1v1 even at lvl 6 cause you can get aquila treads easly before anyone gets boots. Radiance blink is a must for this build for good early game fights and initiation. Also radiance will make your farming easier and make your whole team have evasion. Manta octarine and AC are the best choice for this build, hovewer if they have a lot of evasion, buy mkb. You can even buy bkb if they disable you a lot. S&Y and armlet if you want to end the game quickly.
Mid game alch is way more fun. Quick armlet and treads can be amazing in early to mid game since you can get it really fast. The shadow blade with your W ability will make you a ganking machine. S&Y gives you great stats and slow, BKB for disables is esentional since you dont have much hp and regen as the "Classic" alch. AC if also amazing it will just make your team tankier and their squishier. Also gives some nice attack speed. Last upgrade your shadow blade, or upgrade it earlier if you are fighting like spectre or PA.