I'm not gonna tell ya where I think you should start cause I don't really know you, but I'll say this--if you start off doing something you don't like, you can always transfer.
I can't speak for Entertainment, but Merch can be pretty slow going sometimes and then it can be really hectic other times. I'd compare it to working Merch at a mall, but with a little bit of theming thrown in.
F&B, good choice, don't do it.
Operations--now this is where I think all the fun is. Everyone has their favorite attractions to work and I know a lot of CMs who are retired that work in attractions. Take Adventureland/Liberty Square Ops, for instance. There's physically tiring (Mansion), mentally tiring(Hall of Presidents), and physically and mentally(Jungle Cruise) tiring attractions. Now, imagine being there and eventually being trained at all of them--working one day at Mansion, one at HOP, one at Jungle-- there's sooooo much variety.
The biggest difference between Ops and Entertainment versus Merch and F&B is money. No one's paying any extra to go ride the ride you're working, they already shelled that money out when they walked through the gate.
"Bring me back some dinner, I'm broke. I mean, I don't work. Well, I do, but I'm still broke. I really don't make any sense. I mean, I work here so I don't make any cents, hence why I'm broke."
I hope something made some cents...sense.