Use at the very least a hand crossbow. Why? Because Crossbow Expert is awesome is why. Two attacks not only means more damage, but more chances to hit at all, and thus apply Sneak Attack damage.
It does take up your Bonus Action (which could be used to hide), but say you use it to hide:
You have advantage on your next attack, so you roll twice and take the higher, then deal one lot of damage if either hits.
Now say you use the Bonus Action to attack instead:
You roll twice, and deal a lot of damage per hit.
EDIT: Multiclassing Warlock for 3 levels can be useful if you can figure out at which point you want to do it. It'd allow you to run Hex or, more importantly to a Rogue, Darkness (on self) + Devil's Sight. The enemies can't see through the darkness so have Disadvantage on attack rolls against you, and you gain Advantage on attack rolls against them.
If your DM allows feats, then I suggest
Crossbow Expert, for the reasons listed above
Skulker, because even if you miss with a ranged attack while hidden, your stay hidden, along with...
Weapon Master, for access to the longbow* and heavy crossbow, because...
You can use Assassinate and Death Strike with ANY ranged weapons 
*Or, you can just play a High or Wood Elf, getting free proficiency with longbows.
no crossbow expert SUCKS and here why: the second attack use your bonus action, and when you reach level 2 you gain cunning action that already use your bonus action. and the loading feature of crossbow do not affect rogue, since rogue only have 1 attack per action anyway, rogue never gain a second attack
Here my favorite build for an assassin:
Halfling lightfoot give you:
lucky: reroll all 1 rolled on D20
you can move trought ennemy and friend square that are medium size+ without penalties.
you can hide behind a medium size creature.
+2Dex
Equip a light crossbow so you still do d8 damage.
As a rogue, your sneak attack apply only ONCE per TURN no matter how many attack you do. So to maximize your rogue, you want to have a reliable reaction and a reliable action. Your bonus action is already used by your CUNNING ACTION, so anythign giving you a bonus action is redundant.
So with this build, as soon that you reach lvl 2, you go hide behind a medium size party member with your cunning action. you simply need to beat the ennemy PASSIVE PERCEPTION. So if you put your expertise on stealth, assume a 16 in Dex (+3) that will give you a +7 on stealth roll. On average monster will have 10-13 passive perception. and you reroll 1's so you can very easyly be hidden.
Since you attack while hidden, you attack with advantage EVERY TURN.
Multiclassing 3 level in fighter/champion will make you crit (including your sneak damage) when you roll a 19 and with advantage it will give you a good chance to crit.without loosing much and the archery fighting style for +2 with your crossbow) Second wind is a nice "oh ****" cooldown that will let you survive your failed roll when you go scouting. Action surge can be usefull for when you fail your bonus action to hide or if somehow you miss your target with your first attack with advantage or if the target is weak enough that a non sneak attack will kill it.
Alternatively 3 level in sorcerer will give you a better AC (dragon resilient give 13 compare to a studded leather 12) more HP, and 3 level will give you access to invisibility which is very good for assassin. Subtle metamagic to spell to cast even while hidden and twin metamagic if you want to bring a friend on your assassination attempt
your stat should look like:
Str: 8
Dex: 14+2=16 main stats, do I really need to explain why?
Con: 14 (surviving the poison trap)
Int: 12 (investigation to find trap)
Wis: 12 (perception to find trap)
Cha: 13+1 =14 (to cast illusion and enchantment spell in order to approach your target without arising suspicion, giving you surprise attack: http://www.giantitp.com/comics/oots0767.html)
If your DM rules that trap are found using Perception only then take 8 in Intellect and 14 in Wisdom and with the left over point you can increase strenght to 9 to increase your jump distance and your carrying capacity.
SKill proficient:
Stealth, perception, Investigation, atlhetic (you got a good dex to suceed acrobatic check, but you lack Strenght to succeed athletic check, so behing proficient will help you)
tool: Thieve tool, +2 extra tool or language your choice DO NOT TAKE DISGUISE OR POISONER KIT since assassin give it to you for free.
then boost your dex to 20 ASAP (level 4-8), at lvl 12 magic initiate is also a really nice option, choose cleric, and take guidance and resistance (+1d4 on your most of your checks/saves) and bless that you can cast once per day and keep your concentration on it to give +1d4 to 3 ally attack rolls(including you), then sniper feat (lvl 16) (to ignore cover and be able to add 10 damage on easy target, remember you got advantage on all attack so the -5 is not that bad) (lvl 19 Alert because it will assure that if there is a surprise round you will act in it and that you'll be the first one to do so.
Sorxores wrote:
Yunru wrote:
legendaryhuntsman wrote:
Sorxores wrote:
I made my assasin purely the way I like him and if he isnt min maxed eh whatever thats not what the game is about to me. I suggest you create a picture of him or her in your head and just go with it and have fun with them. Mine is a two weapon fighting kind of guy and as variant human I took the dual wielder feat. He holds a longsword in his off hand and a short sword in his main. So far he is alot of fun
Yunru wrote:
Crossbow Expert is a good pick, provided that you have a party that will back up the choice. With good party synergy and tactics, it gives you another shot at getting that SA damage, which is great. The downside comes from having a party that isn't well equipped to give you the cover and blocking you need. A good chunk of the Rogue's survivability comes from Cunning Action, which plays against Crossbow Expert. Unless your party is well equipped at grabbing the attention of the enemy, and keeping it off of you, you may find yourself getting mulched pretty quickly if you stick around in melee too long.
Feat wise, my favourite Rogue feats are Lucky (fits the the archetype very well and is a great clutch feat), Mobile (Rogues benefit a great deal from high mobility and battlefield positioning) and if your party is built for it, Sentinel (generally more of a tanking feat, but works well to give you another SA).
I like Sentinel as well. Find Familiar is nice too and a good reason to take arcane trickster. Sleep, minor image, and mirror image are just icing on the cake.
There's a few ways I would go with your concept. If you're sticking with Elf, definitely go Wood Elf. I would keep my stats at 16 for Dex and 14 for Wis. That'll give you 14, 12, 12, 8 or 14, 12, 10, 10 to put in your other stats depending on how you want to play your character. Perception is an important skill for an Assassin, so that's why I kept it high, not to mention getting the racial bump to Wis. A high Con will keep you alive longer in combat. Str will make climbing and jumping easier. Int if you want to be more investigative. Finally, a higher Cha if you want to be the disguise type.
Expertise should go into Stealth and Perception. An Assassin is stealthy, but most importantly aware of their surroundings.
Take the Alert feat at level 4. Going first in combat will be your bread and butter. Turning the tide on an ambush and killing one of the ambusher's before they even act is awesome fun. It'll also help to make sure no surprised enemies beat your initiative and foil your assassinate attemp with a reaction. If you're willing to go variant Human, take this at level 1.
Everything else is just gravy. See him as more of a mix it up in combat sort? Take a level of Fighter for Two-Weapon Fighting or Dueling style, medium armor, and Second Wind. Look into maybe taking Medium Armor Master, or possibly the Dual Wielder feat.