I just got into the game in 2008. Knew I would be flying a lot and off I went.

1. AA. Did a PLT Challenge and was EXP within 7 months.
2. CO PLT. Status match of EXP
3. FB Gold, Status match from AA PLT, during soft landing phase of my AA status.

I may land a job in the next year that will get me back to the top of one program or another.

I'll jump off track and talk about Hotels too.

1. HH Diamond. Earned as a conference planner in 2000. Did a Visa/Stay 3 nights challenge in late 2011 for Gold status. Now a Silver.
2. SPG Platinum. Did a challenge in 2010, stayed 48 nights that year. Am now a Gold on stays. Will be a PLT on stays by mid-August.
3. IHG Platinum Elite. Had the in-laws coming into town and hotels were expensive, but I was able to buy enough points at a much cheaper rate to put them up and as a bonus was granted Platinum status.

I was with TWA for a long time.
I then transitioned to AA (remember "More Room Throughout Coach?") after the merger/buyout.
I then was with NWA for almost 10 years before the Delta takeover (I still miss the ease of NWA award booking).
I have now been Platinum on Delta for the last few years, but as a leisure flyer their implementation of MQDs has me comparison shopping - maybe back to AA.

TWA->AA->NWA->DL->???

I started out almost only into AA but that's waned. I've gone in terms of priorities:

2002: AA
2003: AA / HP
2004: UA / AA
2005: AA / UA / HP
2006: AA / UA / US-HP
2007: AA / CO / UA / US / NW
2008: UA / AA / US
2009: UA / AA / DL
2010: UA / DL / AA
2011: UA / DL
2012: DL / UA / AA

Well before I started with FT, I was very agnostic about flying out of SEA:

HP, WN, AS

Eventually got elite status on AS.

Then I learned more, and started playing the game, buying miles (or converting) on arbitrage. So now it looks like this:

AA, UA, WN, US, BA, VX, VS*, IB*, AS, (still elite on AS)

Right now I actually have more *A miles in UA/US than AS miles (thanks to me cashing out about 200,000 miles in the past year on AS, plus 45,000 AA miles).

I'm pretty agnostic on who I will fly in the US, thanks to AS elite status. I will get reasonable room in coach on AA/DL/AS, occasional upgrades on DL, more than occasional upgrades on AS. But I've actually flown UA and US in the past year without elite status and lived to tell the tale.

*miles through Hilton

CP, then AC pretty much exclusively till a little over two years ago when I discovered United flew the same transborder and overseas routes at half the price.

Said buh-bye to AC last year and went with Delta for transborder, and DL/KLM/KE for my overseas travel. I bite the bullet on domestic and use WS ... no real FF program there, but they're generally cheap enough my savings constitutes enough of a reward.

I still like AC, and as a Canadian citizen I would prefer to support a Canadian business ... but they're simply no longer competitive. Nice product, but not worth the considerable premium.

1985-2007: Almost strictly UA. 1P for most of the early 2000s.
2007-2010: CO. Mainly Plat.
2010-2013: UA again, due to merger. Plat/1K.
2013: DL. Did status match to Plat, due to better routings and wifi. Also, without having US part of *A, UA became fairly worthless for me.

Started flying TWA in 1991 and achieved Royal Ambassador status (I loved that you could get free upgrades for both domestic and International segments

In 1993 my wife and I started flying Continental because of their great Elite benefits. I loved the transcon flight where I used to get upgraded nearly 100% of the time.

I started out with NW, LH, CO, and AC (no strategy really, mostly signed up because I happened to take one of their flights). Made elite in AC but switched to UA in 2000 because UA was employer preferred airline. Matched to AA, DL, AS at various times but ultimately decided to stay with UA because of better redemption possibilities and *A partnership airlines like LH, TG, AC, etc. which better fit my travel patterns.

Disappointed with the new UA but I'm not traveling much these days for business, so having 1MM status is liberating and works out well for personal travel. When UA is competitive price wise, Im happy sending my $$ to Smisek, otherwise I just travel LH, AC, AS, HA, DL or whoever offers the best option.

Started with my first job in 1981. Based in DTW so flew mostly NW. In 1984 took a job in ABE that required a lot of travel, continued to fly NW to Asia and Europe but US domestically. Eventually became CP on US but abandoned them for UA when their product went to hell. Was 1K on UA for many years and used them almost exclusively until I retired.

So for me,

NW->US->UA