I think air is the hardest to master: its minions are relatively weak (no strength 4 minions), and most are also low health. Its direct damage spells are not powerful, and most of its other spells require other cards to be truly effective. None of its creatures have special powers that are overwhelming (several are useful, but in subtle ways). And its auras are hard to use well. Moreover, by far its most durable creature (the aerovore) has adverse effects on most of its timed cards. I do not think air is the weakest faction -- it has several nice combos and is a nice complement to most other factions. But without careful design, an air deck will likely be weak.
I think second hardest is forest. Forest tends to be a rather defensive faction -- something contrary to most players style. It has very little to break stalled lanes, nothing for quick creature removal, and nothing to inhibit opponents playing creatures. It is a faction that requires careful thought to complement other factions -- something reflected by its rare appearance in PVP play.
I think fire and ocean are probably the easiest to master.
It is very hard to put together a bad ocean deck -- all cards are very effective, useful, and work well together. Thanks to numerous cards that send other cards to your hand (sea dragons, pearl divers, pirate raiders, triton hunters, sink, sunken treasure, back to the depths), an ocean hand rarely runs short on cards. Ocean has some of the most underpriced cards in the game (triton aquamance, sink), and is disputedly the strongest faction in the game.
Fire has some of spellcraft's preeminent cards: pyrohydra, ruby dragon, meteor. Decks containing those cards are always tough. Most fire cards do not require combos and are straight forward to use. There are fire cards best avoided -- at least in most decks -- but these are not hard to identify.
Underdark, jungle, and swamp are all fairly easy, but have pitfalls.
There is a tendency for underearth decks to be a bit slow. Underdark has a relatively large number of good but expensive cards (underdark worm, taken under, darkling slavers, deepspawn, etc) which, unless balanced by lower cost cards, can make the faction sluggish to respond. Underdark also has by far the best countering cards in the game (primal ooze, essence exchange, mesmer, tentacles from below, dark fey, taken under, etc.) But drifting too far into counter measures can keep a deck from having cohesive offensive ability.
Jungle contains so many tempting cost 3 cards that it is tempting to create decks that do not utilize power points well (I have found I want more cost 2 than cost 3 cards to avoid large numbers of unspendable power points. The ravager is a very tempting, powerful card, but can be very counter productive unless used carefully. The real backbone of jungle power is the giant constrictor. Once players master this card, jungle becomes an extremely formidable faction. It is certainly possible to make strong jungle decks without constrictors, but that is not so easy.
Swamp has the fearsome bone dragon, potentially mighty stitched golems, the unstoppable ancient ghosts, dangerous blood orbs, and the tide changing bodyswap. But, except for the dragons, the cards do significantly benefit from set up and support. Managing discard piles is a little tricky since different cards compete for the same resources, and one cannot see the contents. Swamp is easily strong, but getting the last little bit from the deck requires significant finesse.
I agree on the answer of Quintivarium with the following notes:
AIR: hardest to master - it took me several time to build a decent deck
FOREST: I don't like this faction and never built an effective deck
SWAMP: nice faction but it took me some time to build a deck which works
UNDERWORLD: It is easy but extremely flexible. It takes time to master all possibilities
FIRE: Extremely easy but not that flexible.
OCEAN: The best faction ever. Very straightfoward and flexible.
For your knowledge:
I started with a fire/ocean deck and the first card I wanted to buy was aquamancer.
There are more combos to explain so do not consider the deck as competitive, but I guess that's a good starting point at a reasonable cheap goblinggold price for a new gamer. I started more or less like that.
Goblins should answer that. I can only answer for my case.
I started with a fire/ocean
I switched soon to ocean/fire
I invested my goblingold to buy swamp cards but did not really use them for a long time
I bought then underworld which found easier to master
with the Air expansion I got back to swamp
I won the tournments and I used the gold to buy all missing card, included forest which I still do not use because I do not like it at all
Jungle was obviously the last one given the expansion
If I had to do it again today from the beginning I'd follow this path:
Goblins can be more thorough/accurate, but I believe there are several, different starter decks one could be given randomly. They do use different factions. I believe my first deck was forest underworld. Later (don't ask how -- I don't know exactly) I got ocean and underworld. I think starters all use fire, forest, ocean, or underworld.
Yes, there are quite a lot of different starter decks,with all the 5 starter factions and an equal amount of each faction mix. Originally we had random starter decks but later we changed this as there was some chance you could get a deck that could be too weak for a beginner or had too many unusual cards in.
I agree with Val's analysis on Air being the hardest to play well. It's subtlety actually *requires* it to be harder to play, as compared to in-your-face factions like Fire and Jungle. Air will continue to uncover very cunning combos with future factions too.