Everyone has a set of standard packs that start as $1, then after purchase goes to $
5, then $20, then $50, then $100 then stays at $100 until the end of time. The best packs to buy out of these is the 'Super Gems Deal' which includes a large amount of gems that you can then use to buy just about anything in the game.
During Holiday Event times the best packs to buy are Event Packs by far. These are found by clicking on the Icon on the top right of your screen. Evony does Holiday Events almost constantly with usually only 3-4 days between each event.
The Event Packs reset to $5 every day of the event unlike the regular packs so they are a good way to get Gems on a budget as the $5 pack contains 21k gems! Just simply buy a couple $5 packs each event and you will be set for bubbles and other necessities.
The other items in the packs tend to vary from event to event but usually contain a large amount of resources, speedups, teleports, stamina, monarch exp, and other things. The 5th pack in the series (or ladder as these are often referred to as Ladder Packs if you buy all 5) contains a much larger amount of things than the other 4 and includes a Premium Historic General (one not found in the Tavern), Badges, Dragon Crystals, Dragon and Achaemenidae Scroll chests, and 3 Super Attack, Defense, and HP buffs that increase your troops stats by 50% for 10mins.
During the 3-4 days between Holiday Events is a mini "Civilization Celebration" event that you can buy 30k gems from for $5 and then 60k for $10 if you really feel the need. The mini event packs also contain 'General Momentos' which you can save up over multiple events to buy a general with starting at 30 Momentos.