A3: A bit of a hurdle to achieve *G (12K+2 A3/OA segments or 24K for *S; then you have 12 months to get 24K + 4A3/OA segments or 48K miles for *G).
Easiest to maintain once you reach *G if you are EU-based (12K + 4 A3/OA segments or 24K). Downside: be wary of LH-group flights, which often credit zero (even in J). No Lifetime status.
SK: Harder than A3 to maintain - 45K/year (not really any harder to achieve, also 45K) but much better matrix of earnings on *A partners (especially LH-group, where you don't get hit with zero-earning classes like A3). Lifetime status is possible.
OZ: Seems relatively easy to achieve (40K in 2 years) and retain (30K in 2 years). Downside: poor earnings in general (certainly compared to SK, UA, TK). Lifetime Status is possible. Another drawback (apart from poor earnings): they have what is possibly the worst website I've ever seen for an airline.
TK: Kind of in-between the above 3. Harder than OZ to achieve (40K in 1 year, not 2) but not that onerous to maintain (25K in year 1, or 37.5K in a 2 year period). Good mileage earnings on *A carriers. Downside: your award miles will obviously accrue to TK, and redemptions are more cumbersome than on most airlines. Best route network in the world means you can fly on their metal pretty much anywhere, and it's generally nicer to have earned elite status on the airline you're typically flying.
ET: Good option if you have lots of short Y and not much J. Sheba Gold is 40 segments per year. Note that for mean-spirited LH, 30 segments or 35K miles only gets you Silver - and from next year status will only last for one year rather than two. The cheapest way to earn this is to do domestic segments in Poland. If you go this route, then you will get very well acquainted with both the Dash Q400 and small airports like SZY (Szymany), where staff will probably know you on a first-name basis by the time you've hit *G.
Avoid: LH group, NH.