If it's interesting EMUs you're after, you're going to the right place!
A personal favourite on the RER, the MI84s also dying out on the RER A but still going strong on the B along with their MI79 sisters (but not so keen on the refurbishment inflicted on those examples).
MF67s on Métro lines 3, 3bis, 9 (being replaced), 10 and 12, and the MP59s on the 11.
Z5300s, weekdays only. Juvisy - Melun on the RER D in the peaks every 15 minutes, and Melun - Montereau via Héricy on the Transilien R, quite a nice run and I think they run all day. These are pretty ancient units to say the least!
Slightly less interesting but still worth a bash if you ask me are the Z5600/8800s on the RER C, especially if you can find an unrefurbished one with brown interior. Also the Z6400s running on the Transilien L out of Saint-Lazare.
You're really going to struggle with MS61s I'm afraid - last time I was in Paris in November there were only two or maybe three sets out in the evening peak, and only one out on a Saturday. There's now only six units, so two trains, left in service so it'll be even harder than that! A real shame as I'm quite a big fan of them.
For locos, I'd echo the above posts, also adding the BB17000s which are dying out but still common on Pontoise - Creil (part of Transilien H) and Paris Nord - Crépy-en-Valois (Transilien J - warning, not a nice route to say the least!).