AA23 double suited.

Long answer:

Typically, the game called Omaha High-Low is actually better described as Omaha 8 or better, in which the qualifying low hand has to be at least an 8 high or lower.

Omaha 8 (O8) is a game of the nuts. Unlike in Hold'em, where top pair top kicker can win much of the time, in O8, you will typically need the best-hand on the flop and a draw to the nuts by the river.

Also, since the pot is split among the high and low hands, you often won't have the requisite pot odds for a single draw–you'll need multiple draws to get the right odds.

Finally, you'll generally want to play hands that can "scoop"–that is, win both the high and low ends of the pot. I've seen lots of beginning players who put in big bet after big bet with only the nut low, only to find themselves quartering the pot with someone who has both the high and the low, or worse yet, two other lows and a high. In other words, they've contributed 1/3 or 1/2 of the money in the pot, but only get 1/6 or 1/4 of the winnings.

Therefore, a hand like AA23 double-suited (e.g., Ah As 2h 3s) is the best starting hand because you have the opportunity to scoop with several draws to the nut high (one of two flushes, the wheel, a full house, or quads) and several draws to the nut low.

For more on O8, check out Ray Zee's High-Low Split Poker for Advanced Players from Two Plus Two.