The champions you mentioned don't really have any finicky animation cancels for you to worry about. 
Gangplank and Kennen play more like ranged bullies while Irelia is a mobile skirmisher (so you can close the distance and try to outplay people with mobility) while Rumble is a battle mage with fairly short range.
Hard to make a recommendation without knowing which of those playstyles you prefer. I think Irelia is probably the most likely to be fun, but might be harder to be successful depending on what level you're at. If you're in low elo, you should be fine once you learn her mechanics. You can also play her top, mid or bot pretty well.
Rumble is a strong option for both solo lanes. He has a bit of a weird playstyle so I'd try a few games of him to see if you want to learn him.
You could also try Kled, who you can also play either top or mid. He's fairly simple mechanically and really strong right now. The main thing you have to learn on him is how aggressive you can play without dying. He's a good champion if you want to push your limits constantly and go for fights that seem risky but are actually winning. His ultimate is good for roaming and engages so you have a bit more power to guide your team to good fights.
Maybe try Yorick if you want a split pusher. He's kinda like Nasus but a bit more well-rounded and stronger earlier in the game. For how simple his mechanics are, he has the potential to smash a lot of matchups, including ranged champions. He's not as good in other roles though.
If you want a magic damage champ, maybe try Mordekaiser (top or mid). He's not too hard to play and his ultimate can let you outplay ganks and remain useful in team fights no matter how behind he is. Works well as a split pusher or a team fighter.
That's probably most of the simpler-but-not-braindead top laners I can think of that you might want to look into. If you have a more specific playstyle that you like, it might narrow down the list. There are a lot of viable champions top lane that work well if you take the time to master them, and a lot of those champs have very different ways that they win games.
Gangplank and Kennen play more like ranged bullies while Irelia is a mobile skirmisher (so you can close the distance and try to outplay people with mobility) while Rumble is a battle mage with fairly short range.Hard to make a recommendation without knowing which of those playstyles you prefer. I think
Irelia is probably the most likely to be fun, but might be harder to be successful depending on what level you're at. If you're in low elo, you should be fine once you learn her mechanics. You can also play her top, mid or bot pretty well. Rumble is a strong option for both solo lanes. He has a bit of a weird playstyle so I'd try a few games of him to see if you want to learn him.You could also try
Kled, who you can also play either top or mid. He's fairly simple mechanically and really strong right now. The main thing you have to learn on him is how aggressive you can play without dying. He's a good champion if you want to push your limits constantly and go for fights that seem risky but are actually winning. His ultimate is good for roaming and engages so you have a bit more power to guide your team to good fights.Maybe try
Yorick if you want a split pusher. He's kinda like Nasus but a bit more well-rounded and stronger earlier in the game. For how simple his mechanics are, he has the potential to smash a lot of matchups, including ranged champions. He's not as good in other roles though.If you want a magic damage champ, maybe try
Mordekaiser (top or mid). He's not too hard to play and his ultimate can let you outplay ganks and remain useful in team fights no matter how behind he is. Works well as a split pusher or a team fighter.That's probably most of the simpler-but-not-braindead top laners I can think of that you might want to look into. If you have a more specific playstyle that you like, it might narrow down the list. There are a lot of viable champions top lane that work well if you take the time to master them, and a lot of those champs have very different ways that they win games.
Thanks to Fatelblade for the sig!
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Ps. I know Garen is pretty good now. I might try him, I am also playing Sion now. My ideal champion can impact the game as I do want to climb with them once I reach ranked.