Ah, so many moves lost over the years...
I'd have to say Metronome and Mimic are most missed. Metronome (Generation I TM 35, Emerald Move Tutor) was quite fun to spam. Never worked out well, but it was fun. Mimic (Generation I TM 31, FRLG/Emerald Move Tutor) is only missed in Generation IV due to its ridiculous glitch potential. Who doesn't want a Sp. Atk Salamence with Spacial Rend and Aeroblast? Blame lack of Mimic in Generation IV.
In Generation V, I say the lack of Sucker Punch Move Tutor hurts quite a bit. Now Electrodes can't get Sucker Punch, though it was pretty pointless to start with.
For Generation VI, I'd really want the return of Selfdestruct (Generation I TM 36) and Tri Attack (Generation I TM 49 I think). Quite a few Pokemon can learn Selfdestruct but not Explosion. Mewtwo and Snorlax, for example. And Tri Attack is a decent, respectable move. Oh, and Secret Power (Generation III/IV TM 43). Works great for Serene Grace things. And the best of them all, Softboiled (Generation I TM41). It was so amazing that only one Pokemon was intended for it (Chansey) but a better Pokemon accidentally got the better advantage for it (Mew).
I am a glitcher, not a cheater. I do not alter game codes, but use what's given to get what I want.
Glitches: the solution to the AI's infinite hax. The Battle Frontier AIs may cheat with statistically improbable events happening at every opportunity, but I laugh at their Pokemon team with No Guard Sheer Cold. They may never miss Sing and Grasswhistle, but they can't pull out a Ninjask with Spore. They may never miss Thunder and Hurricane, but they can't pull out a Jolteon with Water Spout and Hurricane in the middle of a Drizzle set. In short, the AI can kiss my a-
As I believe in fairness in battle, I will not abuse the power. In any and all player battles, only RNG is used. No glitches without prior consent.