My money is on a bent valve. If you have enough fluid in the cylinder and try to turn the engine and compress the fluid, if the valves try to open at the same time there can be so much pressure that there literally isn't anywhere for things to go, resulting in excess pressure on the valve stem, usually causing it to bend. If it was broken, you'd hear bits of valve grinding away in your engine, usually.
If it's flooded really bad, you can often put the bike in 6th and push it backwards to try to blow things out. Due to valve overlap and rotational speed, this is often more effective than using the starter if things are really bad.
1986 Yamaha XT350
2005.5 KTM 950 Adventure
1991 BMW 318is turbo track car
1989 Chevrolet Caprice Classic Wagon, TBI 5.7, NV3500 5-speed, lowered, etc...bike hauler