I believe every (Socionics) type of person can potentially "fall hard" for someone, depending on various conditions. How much a person will experience infatuation seems largely be dependent on how much the other person fits your love map, your hormone levels, attachment style and perhaps even Love style. I would say all of these factors add up to the whole experience of "falling in love (limerence)".
You could make the statement that Logical types *generally* fall not in love as hard as Ethical types. However, with the right mixture of love map, hormones, attachment style, etc., I believe some Logical types do "fall harder" than some Ethicals.
Before I suggest what type (combination) would "fall the hardest/least hard", let me preface this by pointing out the fact that the whole experience is subjective. For some people, just randomly thinking about the person and wanting to be with them is already a big sign for loving them greatly. Whereas others only testify someone as "in love" when they cannot sleep and eat due to their obsessive focus on the person. I base my assessment on the general idea of someone "falling in love hard". I list the attributes from the order of most to least importance.
Falls in love the hardest: SX instinct first, Preoccupied Attachment Style, Mania Love Style, Eros Love Style, SX/SP stacking, E2 or E4, Ethical type (esp. ExFx).
Falls in love the least hard: Avoidant/Dismissive Attachment Style, Ludus Love Style, SO/SP stacking, Pragma Love Style, SX blindspot, E5 or E1 (may not apply if SX first), Logical type (esp. xxTj).
Edit: I have come up with scorings. First three aspects count 3 points each, following three count 2 points each, last aspect counts 1 point.
Whatever list contains more aspects that apply to you, this list's scoring will count (more). (You can be a mixture of the two lists, I suppose. It is like the two different sides of you. )
(Also, you may only choose one Love Style as your main one for the scoring.)
If little (1 point) from either list applies to you, consider yourself: Average Joe/Plain Jane. Your Love is "Average".
List 1 scoring:
16-10 points: Love Stalker. Your Love is "Burning".
9-5 points: Hopeless Romantic. Your Love is "Hot".
5-2 points: Cupid's Student. Your Love is "Warm".
List 2 scoring:
16-10 points: Frozen Heart. Your Love is "Freezing".
9-5 points: Pragmatist. Your Love is "Cold".
5-2: points: Love Newbie. Your Love is "Coolish".