I've done tri's and they weren't very exciting, for me at least.
Love crits because the pace/intensity is high at the front and I love sprints-can get quite sketchy tho
Love RR's for the strategy and I know my strengths and weaknesses (not a climber, ouch)
Never done a TT but would guess that's the safest (next to tri's) as far as wrecks.
I do vote for pushing yourself to puke on all of them though-good times.
If you opt for an RR or crit, get some pack skills first. Both provide racing in close quarters but you'll be diving into tons of tight corners with high acceleration after on crits. Either way check with your LBS for rides, races and groups to join.
Good luck.