The upgrade in power to Fly was sorely needed. Defog was generally useless until it cleared out all entry hazards in S/M. Whirlpool (and other moves like it) haven't been useful since Gen1, when they actually trapped Pokémon and made them unable to attack. Cut remains useless, and Surf as as it always was. Rock Smash would probably be my pick since it also got a boost in power while keeping its high chance to lower DEF. And with Technician it's a pretty solid Fighting-type move.
Fly because it lets me get around faster Even if it's more of a nuisance than any other HM, I've always enjoyed whirlpool just for its animation in Heartgold and Soulsilver. There's just something very pleasing about cutting through the vortex of water in front of you, which in and of itself is gorgeous. I also find it very funny how Furret can learn whirlpool. A close second for me would be Rock Climb. Although it takes a stupidly long time to get it Heartgold and Soulsilver, once you have Rock Climb it feels like a lot of the world has really opened up to you. There's a lot of backtracking you can do to get really useful items, meet some new npcs, and discover plenty of new areas. It's also really satisfying to watch the long Rock Climb chains in Mt. Silver.
Surf and Fly I guess. Surf because I find the surfing mechanics to me pretty fun actually, apart from the infuriatingly slow speed in Gen 4, and Fly for quick travel.
Well, like the others before I also like the hm Fly. Rock Climb takes the second place and surf the third. I enjoy the animation of every attack and Fly works like a Teleporter. And teleporters are cool, I guess...
Surf is probably one of the very few HMs aside from Waterfall to actually be useful outside of being mere HMs which is why I've liked them the most.
i like Flash , because i watched some soft lock breaker Flash will weaken your enemy's Pokemons accuracy , which is a creative way to weaken your enemy , heres how it works -Your Opponent's Pokémon used Hi Jump Kick -It Missed , and your Opponent's Pokémon get Damaged Thats how it goes
I actually really like Surf, and I typically don't hesitate to give it to one of my Pokémon. It's powerful and I love the wave animation.
Surf and Waterfall. Waterfall is because you need it to beat the game(A giant waterfall in front of the Pokémon league) and Surf is very useful(Both in battle and exploring)
Mine would probably be Surf, since it's one of the few HM moves that are decent in battle. It's one of the only ones I won't use an HM slave for.
Fly, because it acts as a teleporter and I'm really lazy. It's also decent in battle despite being a 2-turn move. You can use the first round to your advantage by avoiding your opponent's attack.
I don't talk competitive just in general as I play a lot of nuzlocke run where fly actually is useful.
I really liked rock climb. I thought it was a really interesting mechanic and I liked having a reason to backtrack to old areas. Definitely made it more fun to explore the region.
My favorite HM of all time was in HGSS, Rock climb! I have always loved that one for reasons that I still cannot figure out.