I think these are all great. Keep in mind that transform cards, in their limited environment, were created to provide an incentive for people to play spells. If you are looking for a slower more card advantage feel in your format, they may be a poor choice. Some commentary on each of them though:
Cloistered Youth: Good card (edit, thought it was something else)
Delver of Secrets: My favorite card, but will be a poor choice if your cube can't support a spells matter theme.
Ludevic's Test Subject: Another card I really like, but may be a poor choice if there is a lot of removal in your format to kill it, since it punishes people for spending the mana and time to "level up."
Bloodline Keeper: Really well designed card, but may be too slow for some formats.
Reckless Waif: Great aggressive card
Instigator gang: Gread for wide aggro strategies
Wolfbitten Captive: Great aggressive mana sink, but may be a poor choice if you want your green decks to curve out. There is a tension between pumping captive and adding to the board.
Daybreak Ranger: Great card, but may not be high enough power for some formats.
Gatstaf Shepherd: Good evasive attack, great if you are worried about board stalls.
Garruk Relentless: Great card
Huntmaster of the Fells: Great card.