A lot depens on whos limping:

Loose passive- Here I'd raise any decent hand with HU potential (AT+, 88+, anything that might have him dominated) looking to isolate.

Loose agg- I'm limping more hands here, looking to trap if i hit a monster.

Tight - Probably has a PP or SC, You can raise and c-bet a wider range here as they will fold the flop if they don't hit. If they play back at you, fold that TP.

When there are more player in the pot it gets more complicated. Raising a few limpers with 88 or AT can give you some problems if they all call. Just make sure, if you do bet, it will be enough to thin the field. Another thing is to assess the strength of your hand, AJo is good HU so I'd be more inclined to raise, 9Ts is good multiway so go for the limp.