If I already have a card effect turning my lands into creatures, for example: Life and Limb, then I deal combat damage with Liege of the Tangle, and I make all of my lands 8/8's with its ability, which effect takes priority? Do I choose?
The power and toughness are set by the most recently created effect.
Permanent modification is covered by the layer system. Inside a single layer effects are applied in timestamp order, and newer effects will overwrite old ones if they are mutually contradictory.
Thus, if Life and Limb is played after you trigger Liege's ability they will be 1/1s, but if Liege deals combat damage after Life and Limb is added then any land that you add a counter to will become an 8/8.
Oddly enough, this means that you can put awakening counters on saprolings that have been landed by L&L, and they will become 8/8s, and stay that way even if L&L and Liege leave the battlefield.