Hmm.
Most assassins are either mid or jungle. Mostly mid.
Low skillfloor = easy to pick up and play, rewarding, can do well without having to learn too much
High skillceiling = lots of potential, versatility, and various high-level plays that can come with lots of experience
For non-assassin types that are decent at top and/or jungle
You might like Irelia (a toplaner). She's fairly simple, mostly auto-attack based and has a very well-rounded kit that includes almost everything (stuns/slows as CC, sustain, true damage, gapclosers, so on and so forth). She's a bruiser build usually, Triforce being core.
Nautilus is pretty good and getting more buffs soon (not sure why, he doesn't need any). He's an AP bruiser/tank with tons of CC, decent damage, good in top/jungle/support, and has a shield that has AoE burn damage as well as high base damages.
Diana is a pretty strong mid/jungle. She's an AP duelist with fairly strong burst (not to the amount of an assassin, but strong nonethless). She has a 3-bombshield charge, refreshing gapclosers, an AoE pull-in (like a gravity sink towards herself), and an arc skillshot that's fairly easy to land.
Trundle is very strong, has tons of life regen, and his ultimate literally steals stats from the enemy target to buff himself while weakening them. He has strong damage and decent CC and is great in both top and jungle.
Malzahar is alright, although his passive nerfs have hurt him a lot. He's good in top mostly now, because of his ranged poke and his CC denial of most tops (who are usually melee).
For assassin types
Ahri might be decent, although she's not REALLY an assassin. She's really weird. She has an extremely well-rounded kit, much like Irelia, except ranged autos, AP, and much more reliant on skillshots.
Talon's in a sort of rough spot and due for changes/reworks soon, but he's not too shabby and is one of the classic iconic assassins. Katarina is another getting a rework in the update, but she's pretty simple and easy to at least initially pick up, although she struggles in higher levels of play.
If you want me to offer opinions on other champs, feel free to ask me.