Well challenger of course, but we will see if I hit that rank.
What comps do I like? Not sure if you mean what I find strong or what I enjoy to play so I just write both.
I like gunslingers + pirates. They are strong and satisfying to play because thanks to the gold generation and cheap units you can hit a lot of T3s. But I have to say I liked them much more before patch since you had more gold from pirates and not everyone was on the gunslinger + cursed blade hype train( I mean no one was since cursed blade was thrash) so it was far easier to just hit a lot of T3 units. They might be stronger now, but there is not that much satisfaction playing them. If I get spatulas I like to throw in some blademaster Tristanas/Lucians for memes(,,Yo dawg I heard you like procs so I put some procs in your procs so you can proc while you proc").
Personally I like elementalists as my favorite synergy since they are pretty good and you need just 3 so they fit in many comps and the golem is kind of like free slot. And my favorite early game synergy is nidalee + WW. Those two are beasts(pun intended) in early game and they even buffed wild yesterday.
Before the patch my favorite comp was 3 shapeshifters(Nida+Gnar+something) 3 elementalist (Brand+Kennen+something) and some frontline(if possible Seju+Cho). Elementalists did some AOE damage and when Gnar turned he finished what was left of the enemy team(possibly throwing the whole team into ulting Kennen). After the patch most of it got nerfed(both units and synergies) and the game seems faster so I do not play this composition that much anymore since it is pretty expensive and slow to pull off. After the patch I became meta slave and I end running RFC Voli in some iterration(glacials,brawlers,demons) most of the time.