As others before me already said, speed and accuracy are pretty basic factors that you should have a decent understanding of even ig you don't play the mode (it may be relevant that the taiko hit judgement formula is a bit more strict than in standard, not sure if your algorithm needs the specifics. Link to the table in any case:
here).
Pattern/reading skill is probably the hardest part to measure, so I'll give all the input I can give on this topic.
1) SV - Extremely high/low SV is hard to read, but should probably only give a small bonus since DSing is a thing and not provable =/
2) Mix streams - Streams get a lot more complicated if you mix in clusters of 1/3 or 1/6. Not only are they more rhythmically challenging, but they also require advanced hand switching if you fully alternate (which almost all highly ranked players do).
In case you are not entirely sure what I mean, a
ddddkkkkdddd 1/4 stream is really easy since the leading hand hits every white tick while the secondary hand just plays blue tick beats; a
dddd[
kkkk]
dddd stream (square brackets signifying 1/6) however requires a leading hand switch on the second
dddd part since it starts on a blue tick.
3) Pattern complexity - As a rule of thumb, the more repetitive a stream, the easier it is. Streams are especially easy if they only really change on white ticks. For example.
dkkkdkkkdkkkdkkkd... is very easy, so are
ddkkddkkddkkd... and
dkdkdkdkdkdkd...
dkkkddkkdkkkddkkd... is a bit more advanced, but still very basic.
More complicated streams implement
kddkddkddk or
kkkdddkkkddd style patterns - the difficulty here stems, again, from hand switching. While you don't need to switch leading hands offbeat like in the 1/6 example, the "sub-patterns" still force minor leading hand switches (both
kddkddk... and
kkkdddk... are actually non-repeating 6-note clusters even though they look like repetitive 3-note patterns).
All that being said, a pure
kddkddkddkddkddkddk... stream or similar structure isn't extremely hard due to its repetitive nature, it only really gets tricky once you mix it with easier patterns, thus forcing irregular hand switches.
4)
dk kd kd kk dk dk dd dk kd doubles spam and similar shenanigans can be really hard to read and play, at least at high BPM. Again, the more irregular the composition, the harder.
5)
ddddddddddddddddkdddddd... (even number of d, then k) is intuitive and easy to read/play.
dddddddddddddddkddddddd... (uneven number of d, then k; k on blue tick) is really unintuitive and most people will probably miss here on a sightread attempt.
EBAWER already listed some of the best players, so I'll go ahead and give you a few of the hardest ranked taiko maps to check your diff calculator with (also no particular order):
http://osu.ppy.sh/b/179231&m=1 I'm Your Daddy [Fatal Oni] - Unrelenting high speed map with tricky 1/6 clusters
http://osu.ppy.sh/b/92643&m=1 Shinsekai [Taiko] - These streams make absolutely no farking sense, and they last for a long time
http://osu.ppy.sh/b/261205&m=1 IMAGE -MATERIAL- <Version 0> [Firce777's Taiko Oni] - Several long, somewhat complex 260 BPM streams
http://osu.ppy.sh/b/192573&m=1 Because Maybe! pt. 3 [Taiko Collab] - Stamina Killer #1. Patterns are pretty basic, but the speed and length make it incredibly hard to FC/get a good combo
http://osu.ppy.sh/b/245124&m=1 Distorted Lovesong [Taikocalypse DX] - A few of the note clusters towards the end of the map are just stupid at this BPM