Here's a quick comparison of obtaining the first 2 levels of Farming XP (380 XP), let's take some info from the Wiki:
Parsnips:
48 total, 4 days each for
192 days,
960g total Seed price,
~1900g total sell price,
~940g profit
Cauliflower:
17 Total, 12 days each for
204 days,
1360g total Seed price,
~3315g total sell price,
~1955g profit
Potatoes:
28 Total,
23 total harvests (@1.25 Taters per), 6 days each for
138 days,
1150g total Seed price,
~2520g total sell price,
~1370g profit
In this example, you get Farming XP 30-35% faster from
Potatoes. But at the cost of lower profit, more runs to buy seeds from Pierre and more days planting and harvesting.
I'm sure there is a calculator somewhere to maximize Farming XP per time invested but I prefer less effort at all times so I go to
Cauliflower (
Melon and
Pumpkin in later seasons) for the fewer times I need to buy seeds, plant, and harvest and at least I'm getting best profit per tile that way.
Farmed tiles = effort, so fewer tiles means less effort and more time to
doze off Fish lazily at the river.