From the top of my head, irelia/maokai/lulu/mundo/nidalee/rumble/ryze are really good atm. Personally I play a lot of Darius/shyvana/renekton/trundle/jayce. There are so many top lanes that are really good, so it's really up to you to buckle down, pick a few and get decent with them. Like know their pro/con and abuse them. Since you like nasus a lot, and play garen, you should play them even more, or pick up champs like darius/jayce/ryze/renekton (or ap ranged in general like teemo/lulu) because they're really good at both of those match ups (nasus/garen I mean) and knowing your counters is really important to being better at the champion you want to play.
Shen and Singed aren't really played much in this season, since they're pretty dependent at either farming the entire time or farming the entire time. not saying that's bad, but it's definitely not good. Shen mostly sits top, farms, ults, and teleports back top. He splits a lot and is more of a utility player than being damage to the team. Singed is the same way, but he NEEDS the farm because his early game is horrible. It's similar to nasus, but nasus at least spikes up mid game. If a singed is behind early game, he'll probably not make it to mid game. Late game, however, is where he shines so much. Most of the time (as singed/shen/nasus/ or even garen) you're untouchable late game. Definitely keep playing these guys, they're hella worth it, but learn a few other "meta" champs so you can know what to expect from other players.